Yes, depending on what you mean by custom allocator... but this is the same in C++ too, right?
To store custom vertex / face attributes, you can use a variety of sparse techniques to save memory from sitting unused in attribute arrays. Basically you want an interface of get_attribute(type, vertex_id). This sparse matrix, trie, or whatever can be backed by int or float array storage instead of many many tiny malloc'd/gc'd objets.
