It works.  I did this about 6 months ago on my upright.  There are 4-6
pins on an SSIO connector that are used to control the speech board. 
You may not have all of the resistors poulated here (mine did though)
but you can add them easy enough if they are missing.  Just follow the
EDOT schematics (the hook-up diagram with all of the boards) to hook
up the Squawk & Talk board to the SSIO board.  On the schematic,
notice the "jumper" that seems to connect two pins on the same
connector that the S&T board connects?  This is the connection that
tells game that it is in an environmental cab.  If disconnected,
upright.  Just wire a toggle switch here so you can choose between
(full) speech 1 player, and 2 players (with some speech).

I put a small panel inside the coin door with the new toggle switch,
and the volume control for the speech board.  Works well.

As you mentioned you will need a -5V supply (I think that's right,
it's been a while).  My game already had a switching power supply
installed so that was already there waiting for me.

You will also need a special interface for the audio output of the S&T
board.  EDOT has a different dual amp board than UDOT.  Normally there
is an input on the dual amp for the speech.  It is not there on the
UDOT dual amp.  Not to worry, if you look on the MCR2 SSIO board
schematics near C10 and E10 you will see that there was an unused
speech option.  Your board probably does not have the 1uF caps and 13k
resistors installed, so run to radio shack and spend $1 to populate
them (values don't have to be exact).  Then just run the left and
right audio out from the S&Q board to your new 1uF capacitors, and
SPEECH comes pouring out!  Another option (the one I chose) is to
solder those new parts in line on an aligator jumper wire coming from
the S&T board.  Now you can just jumper onto your SSIO board where the
13k resistor would normally go to without modifiying your DOT boards
at all.

The side-effect of this audio connection is that the main volume pot
will now adjust sound and speech, while the speech pot will adjust the
speech relative to the regular sound effects.

As i mentioned above you get SOME speech even in upright mode.  The
switch setting to ENV mode enables all of the speech that would
normally cause the game to wait until speech is finished(i.e.
"Greetings...").  If the speech does not cause additional delay, it
will play in either switch mode (i.e. sark laughing).  The main reason
to even have the switch is because ENV mode does not support 2 players
and UR does not support full speech (hey Tim if you were to hack 2
player into ENV mode or full speech into UR mode, along with your
freeplay hack, I would spend the $26 for your hack).

I've read in several places that the ROM differences for EDOT and UDOT
are because of the speech differences and 1 player only in EDOT. I
don't think this is the case at all.  I have found that my UDOT roms
do everything correctly as if it were EDOT when the switch is in ENV
mode, including speech diagnostics.  I suspect that the control
signals for the strobing lights also work perfectly, although I have
not tested them (after all it is the same data bus that is controlling
the S&T board).  It is probably just the scoring information in
attract mode that is different between the two versions, which are
probably actually just different revisions.