With any plugin you can hook FM_CreateNamedEntity, FM_CreateEntity
With orpheu you can hook engfunc() then filter EngFunc_CreateEntity and EngFunc_CreateNamedEntity.
You can't hook create_entity for now because there is a problem between engine (hlds) and engine (amxx module) library names, unless Arkshine has fixed it but i don't think so.
So yes, you can hook entities creations, and you should be able to hook entities removal as well.
Anyway, such an api seems to be a waste of cpu/memory if it is done in a plugin, and should be done in a module.
Also, i don't see how such an api would be usefull, could you explain what you have in mind ?