Alt as meta, except for certain keys, how?

[Jussi Piitulainen <jpiitula@venus.ling.helsinki.fi>]

Hello,

I'm tearing my hair off.

I want to use Alt as a Meta key, with the exception that certain keys
should be interpreted as ordinary unmodified characters: Alt-7 should
produce vertical bar |, Shift-Alt-7 alias Alt-/ should produce
backslash \, and there are a few others, all in ASCII.

I have managed to (global-set-key "\M-7" "|"). That works for writing
but is not a real solution. At least in an incremental search Alt-7
has already been interpreted as two keys: the escape terminates the
search and 7 is inserted in the buffer.

I could not work out how to use keyboard-translate to turn Alt-7 into
vertical bar. If this is possible, could someone please show me the
concrete command? Or any other way to achieve my goal?

Emacs is 21.3.50.6 from 2002-11-27, a binary distribution for Mac OS
X, the keyboards are Apple's - Finnish layout - Alt is sometimes
called Option. I know I lose a couple of command bindings; I can
produce all accented letters with dead keys.

Thanks for any help,
-- 
Jussi