When Bernie Greenberg did EMACS for Multics, he had a
similar issue. I
recall reading a document with an extensive discussion of how they dealt
with this ... If anyone's really interested in this, and can't find it
themselves, I can try looking for it.
I got a request for this; a Web search turned up:
which covers the points I mentioned (and more besides, such as why LISP was
chosen). I don't think this is the thing I remembered (which was, IIRC, an
informal note), but it does seem to be a later version of that.