Thanks for the responses, it's much appreciated.
I should probably have mentioned I was interested in using RP for role playing purposes rather than as an effective form of government (heh heh, RP for RP). I had initially been planning to try and iron man it, limiting my demense to the province of Byzantium itself- but that's a little too suicidal: even though it's the richest province in the game, it's far too easy to get the province looted and be left with a tiny army.
Thanks for the link Tempest, I was sure I'd seen something like that round here somewhere. I figured the +1 loyalty bonus for being part of the Byzantine empire would ofset the loyalty penalty for this form of government, but the events have been killing me. Previously I've played as Germany from the 1066 start date and never had these kind of problems, even when excommunicated. Does anyone know if the loyalty events tied to RP are done on a by vassal basis or on a by kingdom basis? (i.e. will consolidating my vassals, making archdukes with 3/4 duke titles, mean I get fewer rebellions, or will I be left with just as many rebellions only now with bigger and harder to stop vassals?)
Having said this, last night's attempt seems to be going ok, I've managed to keep the empire intact dispite repeated rebellions (except for the bit up in the Crimea, I've left them to their own devices after they peacefully broke away, it costs too much to get up there and the tribe of Cuman is looking a bit scary right now). I've been steadily replacing the dukes with family members which seems to have eased the number of rebellions somewhat. I've managed to vassalise Cyprus, Antioch and Armenia and get relations installed in the first two of these. I even managed to nab a couple of tasty provinces from Bulgaria when they tried to jump me in the middle of the civil war. The succession has gone off relatively peacefully (only 3 rebellions and the seemingly obligatory realm duress) and I may actually be in a position to expand (probably against the sultanate of Rum, although Bulgaria could be another option).
@ Nick B II
I tend to use exactly the same strategy as you. I do generally just use vassals armies to fight, primarily using mine just to defend against seiges and so powerful but happy vassals are definitely a very good way of running a kingdom. I thought Byzantium in the 1187 start might be small enough to cope with RP (they've lost a lot of ground since 1066), but it seems to be pushing it, especially if I want to expand.