FWIW, I usually use code warrior, which is really more of a C/Pascal compiler, but you can define asm functions, and it takes care of the glue of turning it into an application. I've also used THINKC, which can also do assembly functions, and is probably more appropriate for minivmac.

Another trick I use is to build code on a modern system using sc68 (http://sc68.atari.org), and then use a hex editor/resedit to paste the machine code into the macintosh binary.

Not exactly what you were looking for, but thought I'd chime in since you didn't get any other responses.