I believe it would be easier to provide access to Obj-C's API which
can query out this data. I've found that by exposing the raw API to
Scheme, you can avoid having to write a bunch of translation code
which awkwardly constructs Scheme data structures from C.

Have you look at the code for the Objective C FFIs in PLT Scheme

and Chicken Scheme?

They use a general approach using the introspection facilities provided

by the Objective C runtime. This reduces the pain of writing

bindings considerably.