Per-file word abbrevs

[<Henry at MIT-OZ>]

Rather than having per-mode word abbrevs or global word abbrevs, 
usually what I really want is per-file word abbrevs. I use word
abbrevs extensively for names of functions and variables in Lisp 
code, or commonly typed words in papers. Per-mode word abbrevs aren't
right, because if you're editing two programs their word abbrevs will
clash, or if you're editing two papers. You also need the ability
to import abbrevs from one file to another. Sometimes with Lisp code
what you really mean is that the abbrevs are per-system, but per-file
would be good enough if you could import the abbrevs from the defsystem 
file, say, or perhaps defsystem ought to have a mechanism for this.
Also, using Meta-X Read/Write Word Abbrev File explicitly is a pain, it should
happen automatically with C-X C-F and C-X C-S [which I tried hacking but the
lack of per-file abbrevs didn't make it win].

What do you think? How easy would this be to do?