Thanks for the pointer.
This note in the documentation for that egg looks worrisome [1]: "IMPORTANT: This only works when the code will be run on exactly the same platform as the Scheme compiler ran on. Cross-compilation and compiling to C and compiling that on the target platform is not supported. (You will get an error message when you try to do it anyway)"
Does this mean that cross-compiling for iOS would preclude using this bignum egg ?