Why not...allow people who are limited to one school (either because they have 4 legs or made an RP choice too) use the excess xp after 100 to buy training points or abilities.
Perhaps
1 tp for every 50k xp
or...
12 per xp equivilant to that needed for level 100
or
1200 for xp equivilant of xp needed 1-100
Have Epic abilities that can be bought at the cost of several hundred tps
Eg
Reinforce scales
The user utilises their knowledge of primal to imbue their scales
150% base armour
200% all benefits granted from scales
last 2 mins
recycle 5
For biped prestiges they would need to drop requirements just for simpilicities sake.
The downside (there always has to be one) rating increases on a similar level to that of a multiclasser. As dragons can tell you there is no penalty for multiclassing so why not let them share the joy as well.