There is really nothing to do something like that at the moment.
I can think of 2 ways of maybe doing something:
1. add an "assembler" toolchain, and compile the asm files using that into a separate library, and link this with the cpp code at the end.
2. extend the BuildTool.hx logic to allow a different "compiler" per file-group
Neither option is trivial, but could probably be made to work.
Hugh