I get the public API argument, but it certainly feels backwards to structure the internals of your application for that specific use case. Generating and mapping an UUID to your identifier when you need it is pretty straight forward and allows you to avoid exposing the identifier at all.

Which leaves partitioning, something that very few applications will ever need, even if plenty of developers hope they will. A slightly less weird use case, but perfectly doable up to a point using regular sequences.

I see a solution looking for problems.