OpenEuphoria

You could write an asm stub that takes the parameters from the caller, add some additional information (like a routine_id) and calls a routine that dispatches the calls. Look in the archives for fptr.e to get started. It was originally created to be able to get callbacks that take more parameters than euphoria allows. In this case, you'd have only one real callback, but potentially lots of others that you would be able to allocate or free however you wanted.