I always found it odd that the GEM environment supported this kind of cooperative multitasking by default only for desk accessories. The API support for doing it more broadly was there, but the desktop and all the applications were written so they would not be. So even when we got proper multitasking support under MiNT there were few/no applications that behaved well in this scenario.
I never saw uemacs compiled as a DA. That would have been a nice trick. The only microemacs I used on my ST was not a windowed application, was console only.
It wasn't a trick to compile mgemacs as a desk accessory, it took a bit of work. I still have the SH204 HD from back then so have the source code, just don't have a ST to read it.
I chose the MicroGNU variant as it did parenthesis matching better than the alternatives. I ran it alongside Franz Lisp that I had also made into a GEM application.
I built MicroGNUEmacs as a desk accessory so that I could run it at the same time as other programs.