Has anybody ever put a 1990 ECU into a [1991-1994 DSM]?

This works - kind of. Read this post by Todd Day (of Technomotive) for the results. Technomotive recommends that only 1990 ECUs be used in 1990 cars, and that only 1991-1994 ECUs be used in the later cars.

However, you can make the switch, if you want to. Mike Hamilton has a FAQ on installing a 1991-1994 ECU in a 1990 DSM. This may be an option for people who have difficulty finding a 1990 ECU - they are becoming increasingly rare.