Oh, the answer is that Zortech C is actually open-source, and I was just being an idiot? That's wonderful! And the license here doesn't seem to have any of the questionable bits in the OpenWatcom license.

I wasn't able to make it fully open source until a few years ago.

And maybe people just haven't noticed? I assumed when you said "free of charge" that you implicitly meant "but not open source", and also didn't see anything on https://www.digitalmars.com/download/freecompiler.html about compiler source, and the only "license" link on that page goes to https://www.digitalmars.com/download/dmcpp.html which says, "The Software is copyrighted and comes with a single user license, and may not be redistributed. If you wish to obtain a redistribution license,...", so maybe you could see how I got the wrong idea!

Yeah I need to fix that. Thanks for poking at it.

I'm glad my comment was useful! I was worried it might annoy you.