"but the skeleton function does not contain asm"

Unless I've just misunderstood again

The sole purpose of the skeleton file is to create a skeleton. The first thing you do is to take all code out. Then you insert your assembly code.
Since you take all code generated by the compiler out, who gives a hoot if it is optimized.

A possible crossing of wires may be that I am talking about generating a sourcefile for the asm ONLY code, of course, if you are doing inline assembly it is another story. The only way I mix assembly and "C" is by generating pure assembly functions, after all, if you need the speed/compactness you will make the whole function asm I can't see anything less making sense.

To restate: generate a skeleton "C" file with the function(s) you want to convert to assembly. Rename the generated file .asm .a51 or whatever your assembler likes. Column edit the object out. Edit all instructions out. Insert your assembly code now that you know where and how the parametres are passed.

Erik