Right. I know if you don't do the process buffer code, you can freeze things. And I can fully cede that doing that for elisp is weird. Same idea in a sly buffer, though, is fine.
sly is easier, there the computation is done by a remote Common Lisp. But the danger may be long running output operations, unless they explicitly give up control for other (possibly green) threads.
But in a Lisp application to not be responsive at all is kind of rare. Most Lisp application runtimes will be able to process interrupts and/or switch "green threads" or "native threads".
My Lisp Machine emulator for Symbolics Genera is still responsive for the same code snippet, entered in a Listener window.