Thanks for everyone, especially Herzen's love-child and LordKalvan, for all of the ideas which have been suggested in this thread. I'm still writing lots of notes and trying to turn them into more specific ideas for what may be a timeline (or more likely, a mini-timeline).
In general, though, it sounds like there are several ideas / requirements, some of which may be somewhat contradictory:
(i) Get the Venetian elite to stay more interested in commerce, rather than landed estates in the Venetian hinterland; though this may contradict the next idea, which was
(ii) Build up an earlier and greater sericulture industry within the Venetian hinterland, to act as a supply of fibre for high-end silk textiles which will initially be created in Venice itself, but then the techniques will diffuse using water-powered mills along the Po Valley and other suitable waterways;
(iii) Whether through better diplomacy (i.e. alliance with Milan) or more focus on commerce, keep Venice from getting too bogged down with maintaining territory in Italy itself, ideally leading to
(iv) Finding a way (diplomacy? neutrality?) for Venice to avoid having regular armies moving destructively through its hinterland, and thus wrecking any proto-industrial enterprises and accumulation of capital in those regions
(v) Through whatever means, Venice needs to keep up some involvement with Egypt (the Mameluks), to avoid having the Ottomans become an overwhelming pressure in the eastern Med, and also for better access to markets.
For the particular fields of technology, well, metallurgy, textiles and agriculture all need some boosting, one way or another. For metallurgy, yes, the blast furnace sounds like it's the key. Whether that means an earlier spread from the Low Countries, a direct adaptation from the Catalan forge equivalents in Switzerland/Germany, or acquiring knowledge from China or South India, I'm not sure. Having one of the Chinese texts on metallury make its way to Venice earlier would be great, although I'm not sure whether that would rely on being its own PoD, or if it could be a plausible butterfly of an earlier PoD (e.g. supporting the Ambrosian republic, freeing up attention from the western border, then refocusing their efforts in Egypt, leading to greater trade there, including eventually some Chinese texts).
For textiles, I'm still not sure whether it would be better for Venice to have its own supply of fibre (sericulture in the hinterland), or rely on imported fibres (wool from England). The OTL textile industrial revolution used imported fibre, of course, but perhaps an alternative would be for that silk to be produced in the hinterland, then "imported" to Venice, where skilled workers there produced fabrics for export. With skilled workers in short supply in Venice itself, and with silk having rather a high demand value elsewhere in Europe, there may be enough wages pressure to avoid the guild trap, and allow investment in innovation. (If that works, then I'll be looking at early adoption of the technologies developed in the French silk industry later, for inspiration).
For agriculture, well... I'm not sure, really. Perhaps as a spin-off of better metallurgy (however that's achieved), there's earlier invention of iron agricultural tools, and maybe even the seed-drill. I don't know enough (read: any) about northern Italian agriculture during the period to know whether there's other crops or technologies that could be imported (or spin-off developments of metallurgy) to allow for improved agricultural productivity. I don't even know (yet) whether northern Italy had much in the way of crop rotation at this point.
One other general topic I still need to look into is canals. What are the prospects of Venice building earlier and better canals throughout their hinterland? This would help with both transportation and agriculture, access to markets and so forth, if it's feasible.
And on a general note, a Venetian Industrial Revolution does not, of course, have to look too much like our one. A combination of silk textiles, hydropower and better metallurgy (but limited use of coal) won't develop as far or as fast as the OTL Industrial Revolution went, but it would still be interesting to explore.
For a specific PoD (thanks LordKalvan for the options), I lean more toward the support of the Ambrosian republic. Partly because it seems to give the most options for later development, and also because this would have a convenient link to the name I have in mind for if this turns into a timeline (see below).
For some of the particular topics which people have raised:
Do you still have a link to the soc.history.what-if threads?
Most of the threads can still be found by searching for "East Meets West" in google groups:
https://groups.google.com/forum/#!forum/soc.history.what-if
Another way to get an industrial revolution kicking in Venice is if you can keep it relevant and powerful later than it did, so it could ride the coattails of an Industrial Revolution that has sprung up elsewhere similarly to the time and place of the OTL.
Could the Ottomans not have used middle eastern oil and coal from the Balkans and iron deposits from Algeria, Syria or Turkey to start their own industrial age?
While both a later surviving Venetian Republic and an Ottoman Industrial Revolution would be interesting topics in themselves, I'm specifically interested in any potential for an indigenous Venetian Industrial Revolution. If it can be achieved, great. If not, it will still have been an interesting exercise in exploring the possibility.
Jerad. Getting any ideas from this? Were you considering authoring a TL or leaving this a thought experiment?
Plenty of ideas. Potentially this could turn into a timeline, if it turns out to be plausible. I already have a timeline name in mind:
The Arsenal of Democracy.
If I do write a timeline, though, it will be deliberately shorter and more focused than my previous (and currently ongoing) timelines. Probably a dozen or so focused posts on the development of a Venetian Industrial Revolution.