Thanks for checking on that, Paul - I made a mistake in thinking that if I'm dual-stacking V4 and V6 I'd eat up two VRRPs, which I don't - I only use 2 VRIDs on the same VRRP. D'oh! 511 distinct VLANs, dual-stacked V4/V6, and I can play default-router(VRRP enabled) for all of them - yes, I think that's enough (i.e. after that many, I should get a new set of 8800s).
Sorry for the confusion!
Frank