It's been 5 months, but I can finally confirm that it can speak! Thanks to some help from gamatron2000 and generous use of a machine from drgnlair .
How much work is involved depends on the MPU board used with the conversion kit.
J1+J2 are straightforward since they just pass through the connection from MPU to lamp matrix.
J3 requires 4 connections (double check the connections for your specific game just in case!).
J3-1: General illumination. The obvious choice was to tack this onto the hot side of the GI lamps which power the backbox.
J3-2: Key, no connection
J3-3: Voice out. This normally goes to J3-5 on the SB-300 sound board. On the Pinstar board, this pin is probably not connected to anything. It needs to connect somewhere on the junction of C14 and R23. Of course, Pinstar decided their board did not need a silkscreen , so these parts are unlabeled. See the photo for how we ran the green jumper on the backside.
Pinstar Jumper Location
J3-4: Lamp Strobe 2. Goes to J1-8 on the MPU, and with any luck it is not used! Just crimp a pin on the wire and slide it in.
J3-5: Lamp Interrupt. This is normally J4-11 on a MPU-200. On MPU-100 and Bally AS-2518 based boards, this does not connect to J4!. Instead of pulling the board out to run a jumper we soldered a wire to the high side of R24. Yeah it's not a great solution, don't flame too hard in the comments.
Honestly, the hardest part might actually be obtaining a working speech board. We built a custom replacement, the VSU-2 (for sale soon??).