What was on my mind when I read this (so as I did understand it) is a thing I was thinking long time ago:
I think Mark was asking for tool that attaches some code infront of the MSA file, and makes it executable without copying to diskette (i.e. some sort of self-extractor in memory)
I was thinking about a small reset-proof tool that loads the MSA file on real ST, kicks out drive A: and emulates the drive A: from the the MSA file (something like emulators do, but you wouldn't have to write the MSA file to diskette and could run it from hard-drive...