MacOSX was released in 2001, so it would have had to have been System 7/8/9 by that point.
That is a really interesting what-if scenario, but I don't think the mid-90s Apple corporate management would have been willing to go through with it. At the very least they would have tried to lock it down to Apple designed Intel hardware, they've never been able to let go of being a hardware company.
Give Dell an outdated OS, and launch a brand new OS yourself