It isn't either of these, but boy they sure are exactly what I was describing. I think the more I look at Master System footage the more I believe it couldn't have been a Master System game. The closest maybe I've found to date are Neutopia 1 & 2 on PC Engine, which both very much look like the game I remember, but the game I saw was at my Uncle's and I just can't imagine him ever having owned a PC Engine / Turbografx when I know that he at least had had a Genesis and possibly a Master System at some point.