http://forums.alliedmods.net/showthread.php?t=54117
This link describes how to allow for more than 512 entities on the map.
If you're looking to change the precache limit, like your title suggests, then the above link is useless. I don't think there's any way to change the precache limit.
Also, as far as I know, there's no dirty method for using models without precaching. Maybe someone with more experience can give you a detailed explanation.
As I stretch my imagination, I think I can see a way to use more than 512 models, even though only 512 have been precached. It's awful. You could, for example, make an "animated" sprite with hundreds of frames that would contain all of the other sprites. That way, you could consolidate all the sprites into one massive sprite and control which frames are displayed with an AMXX plugin. This means that you would precache one very large sprite instead of ~30 (I don't know how many) small independent sprites. I suppose you could use a similar method to embed several different models into one .mdl file. I've never tried such a method and it's just an idea so it could be complete bollocks in practice.