Custom allocators?

This thread from a few years ago says it's not possible. I assume that is still the case. True?

I probably don't need it at the moment. I wrote something that uses persistent data structures to model rich text documents. I like how it works - undo and redo are trivial. But there is a potential for a lot of allocation of the same few "document node" types over and over, as the user types and edits a document. So I was wondering about this.

Rob