Is there any chance that the Austrian government might accept (pre 1858) a proposal that has a more or less self-governing Kdm of Lombardo-Venetia; with a "traditional" government with representation of the local Italian nobility, but no explicit parliament?
Vaguely similar to the early modern Imperial Circles, there would be a Italian Confederation, with a clerical power (the Papal States) holding the chancellorship and a temporal power (the Kdm of Lom-Ven) holding the military leadership. Sardinia and Naples would play a secondary role with some rights reserved. Parma, Modena and Tuscany would be more or less direct Austrian puppets.
IOW, Vienna would give up some direct control over L-V, but hope to gain more indirect control over Italy as a whole.
Is there any chance to get something like that already in or near 1815?
Before 1848, it's hard to see why Austria would have any interest in such a thing. They control Italy without it, so what need have they of a confederation? After 1848, it doesn't fit very well with Schwarzenberg's neo-absolutism, but I suppose you could imagine an Austrian regime that creates some form of constitutional devolution in Lombardy-Venetia. Certainly Archduke Ferdinand Maximilian was interested in such a project. It's hard to see how you get the Confederation, though, because, again, it's not really in anyone's interest. Austria dominates the peninsula without a confederation, and a confederation dominated by Austria would not be in France's (or Piedmont's) interest.
I wonder, though, what might happen if the Austrians play the approach to war in 1859 a bit differently. They actually had Napoleon III over a barrel, because he wanted to start a war with at least some sort of fig leaf that he wasn't the aggressor, but he didn't have such a fig leaf until the Austrians sent an ultimatum demanding Piedmont disarm. If the Austrians hold off from the ultimatum, instead of a war, you end up with a Congress of the great powers dealing with the Italian question. That might be your opportunity for a confederal solution. Even there, though, it's tough, because the OTL Austrian leadership is basically just incredibly stubborn.
OTL, they had a very pro-Austrian British government under Derby and Malmesbury basically telling them just not to do anything stupid and everything would work out, and a reasonably friendly Prussian government that would be very hard-pressed to avoid supporting Austria in a defensive war against France, and they piss the whole thing away for no good reason.
Here's a possibility: kill off Franz Joseph some time before Rudolf is conceived (i.e., late 1857) and make Maximilian the new emperor. He's *much* more liberal and (for a Habsburg) pro-Italian. Much more likely to make concessions. I don't think you're going to successfully change Franz Joseph very much, but his brother could easily pursue very different policies. Certainly it's relatively easy to see him avoiding the 1859 war, and much easier to see him supporting reform proposals in Italy.