Note that this stuff requires mg3beta4 or later; and may require
beta5.

mg-startup sets up the spell-word; spell-region and spell-buffer
commands. Spell-buffer invokes the spell-region rexx script with an
argument to trigger doing the entire buffer. The interface is pretty
much the same for all of them - it tells you what word wasn't
recognized, and asks for a replacement. A newline leaves the word as
is; a "?" either gets the same question, or a list of alternatives. If
the list is presented, you can enter the number of the word in the
display area, or type a word in yourself. The buffer then gets
changed.

Note that spell-region was largely done as an exercise; I don't use it
very often, and it hasn't been thoroughly tested. The problem is that
mg doesn't have a query-replace interface for rexx, so the
spell-region just changes text blindly. I don't recommend using it as
is. Watch for mg to be improved, and for that script to become
useful.