Don't know for sure. There is this on Systen 6 heaven:
"When Apple finally released System 7, it caused a huge outcry among developers, as many existing applications were incompatible with it. Ironically, Apple's own development system, MPW 3.1, was one of the applications that was incompatible with System 7. It took quite a while for MPW 3.2 to be readied and released, which allowed native System 7 development until then, you had to develop under System 6 and then reboot or transfer the software to a machine running system 7 to be able to test your software. Adding insult to injury, Apple had emphasized making sure that your applications were "System-7-ready" at three consecutive World-Wide
Developer Conferences ('89, '90 and '91), but their own application missed the mark. Of course, at each of these conferences, they also promised that System 7 would be released "later this year". They got it right in 1991."
which kind of implies that it will. There is also some language in the "About E. T. O. #14" that implies it will run under System 6 but I
no longer have any System 6 machines, so I can't test it.