(Basing the post essentially on
Wales and the Britons 350-1064 by T.M. Charles-Edwards. If you want to point problems with the thesis or book, I'd be glad to hear them)
Mercia, even in this period, was systematically interested on what happened in Wales which was in what you could consider its "predatory sphere", so non-intervention is out of the question.
We know that Ceolwulf, as a client-king, intervened twice beyond Offa's Dike in 877 and 878, showing willingness and capacity to undergo campaigns in the region. In addition, Rhodri had in 877 to deal with "Dark Foreigners" meaning the new Vikings in Ireland and England (Ui Imair) possibly in retaliation against Orm's death.
As long as Mercia (regardless as a client kingdom or an Anglo-Scandinavian kingdom, even if the latter is relatively unlikely at this point) is a thing, Rhodri would have to deal with it on a regular basis.
Now, does that means Mercians were unbeatable? Of course not, would it be only because Rhodri's sons defeated them in 881, prevented Mercian kings and earls to enforce their dominance over neighbouring Welsh principalties, especially thanks to an alliance with Wessex . (Alfred preventing Aethelred to strike back in the west the next year) which saw the acknowledgement of its hegemony over Mercia but also in part of Wales especially in South-East but broadly over all the region.
Basically, it meant that Wales switched from Mercian overlordship to a bit more remote West Saxon one, which was pretty much clear by Asser : "on the same conditions as Aethelred and the Mercians") which is understable giving that Viking raids were still an increasingly present problem until the XIth and that Alfred looked like the sole alternative.
At this point, whoever dominate Wales, Mercians, West Saxons or Anglo-Scandinavians; they would have a similar perspective on things that Welsh hegemons would be unable to reallt deal with. IOTL, Rodhri's successors had to play the balance game between Wessex, Mercia (which recovered its influence over Gwynned and Meryfinion) and Jorvik. I don't think a surviving and temporarily victorious Rhodri would be able to prevent this at all.
Hywel the Good's rise in Wales have to be understood along these lines, being tied to Aethelstan's influence and early submission of Hywel to his authority and ruling as a sub-king (in a complex situation where distinction between originally Mercian and West Saxon influence and its aristocratic translation wasn't disappearing).
So, what does it implies if Rhodri defeats Mercians in 878?
First, that he'll have to search for an ally and eventual protector : there's basically Jorvik and Wessex, depending on how it unfolds differently or the same than IOTL to help them against raiders.
And even in the case of a victorious Anglo-Scandinavian side at Eddington, I don't think Jorvik would be this much of an obvious choice except for a grim scenario for Wessex and giving Mercia is still a client kingdom of Anglo-Danes...
So we would be talking of a personal hegemon over Welsh petty-kingdoms with the more or less genuine blessing of an English kingdom, comparable to
Hywel's own predominance. And, likewise, the overlordship would be split among his dynasty after his death : not only because it was
customary and institutionalized (putting thing simply, it was expected) but because the English overlord would frown on a too powerful sub-king that he wouldn't have "made". While it allowed dynastic stability and transmission, it made the technical ruler basically a high-king IOTL already.
As for expansion, even if all things goes well in the establishment of a personal and dynastic hegemony, there's no real way I could see Rhodri not only taking all Welsh petty-kingdoms without a massive reaction from either Mercians or West Saxons, let alone conquer part of these kingdoms; but they would be busy enough dealing with rivals for hegemony as well as Irish Sea raiding.
An early Abreffraw's dominance over Wales would have impact, possibly an earlier institutionalisation of Wales petty-kingdoms, but it would ask for further PoDs in England IMO (or an earlier PoD) to really make it go off the ground in the long term. I'm sorry if this looks like a too negative answer, but truth to be told, English influence at this point was firmly established, as well as Welsh customs.