The POD does seem plausible, but it would take much to achieve it. Having Louis the German's expedition against Moravia in 864 fail may benefit Rastislav's position. Carloman succeeding in his revolt in 862, during which he allied Rastislav, may also benefit Great Moravia, though in what way I'm not sure. If that doesn't work out, having Carloman's and Charles the Fat's engagements in 868 fail as well would help. Eliminating Svatopluk, Rastislav's nephew, would also possibly work, for he ended up entering in negotiations with Carloman without Rastislav's knowledge, and accepted Carloman's lordship over his person and his realm, thus setting himself up as his successor; so killing him off either before he does that or having him not be warned over the conspiracy to kill him, leading to his assassination, would work out well.
If we assume Rastislav's rule stays stable enough and successful enough against the Franks, then he'd more than likely have his son (which we could assume he had, possibly counting Bořivoj and Gorazd, albeit unconfirmed) succeed him. Unlike with Svatopluk, we wouldn't see a conspiracy form against Methodius and his disciples, preventing the promotion of the Swabian priest Witching as Bishop of Nitra, and thus, after Methodius' death in 885, preventing the banishment of Methodius' disciples and those educated at the Great Moravian Academy founded by Cyril. Since there is no conspiracy influencing the Pope's views and opinions on what's going on with the Archdiocese in the region, as long as Methodius' successor continues to toe the line he did, then there shouldn't be any moves made religiously. Assuming things continue developing quite dandy, those educated at the Academy may head north towards Poland and help in TTL's Baptism of Poland, and since they'd be against the German clergy, they'd more than likely follow the example of the Moravians.
The side-effect of this means that TTL!Cyrillic develops in Moravia or Poland rather than Bulgaria. Though linguistically things weren't much different yet, you know how those butterflies work.