A couple of things I can suggest looking at, or at least eliminating
from the list of possible causes:

1. First, Can you give some further description of the character of the
squeak?  Is it in the frequency range of a door hinge?  Higher?  Lower?
Any other similar sound you can compare it with?  If it is truly a
"squeak" and not a "clack", the position of the key frame into or out
of the keybed is probably not the cause, but if there is a "worm screw"
adjustment (probably in the treble cheek block), it wouldn't hurt to
experiment a little with that positioning -- just make dang sure you
take note of _precisely_ where the screw is before you start adjusting.
If you can't make the squeak any better within one revolution of the
screw head in either direction, put it back in its original position
and keep reading.

2. Check to make sure all of the thin deck-to-deck support wires are
in place and their screws are tight.  There should be at least three
of them, usually positioned in the pneumatic "breaks", about 1-1/2"
long and 1/8" diameter, with looped "eyes" at each end to receive the
screws.  Look for empty screw holes in the deck edges toward the
striker pneumatic open ends.  If there are any, that is probably where
(missing?) wires were.  They are easily fabricated from heavy
coat-hanger type wire.

3. Each deck should also have a horizontal "stiffener" bar of steel
with rounded ends, about 3/16" thick, 3/4" wide and the length of the
decks.  (I'm currently recuperating from open-spine surgery and cannot
get to my bone yard to check these exact dimensions).  Make sure these
are all present and all their screws very tight.  Correct any defects.

Let us know the results.  Good luck.

John Grant