How easy would it be to add I/O for an optional on-screen scoring add-on board?
I think onscreen digits would be doable with a diode matrix for characters,or you could do LED displays (minitron filament displays for period accuracy), or nixies. A long time ago ISTR seeing an idea from the 60's for vector digit generation using various LC circuits to combine sine & cosines to form digits for vector displays.
You'd just have to (for a non-"on-screen" score counter) break out the "Paddle hit" and "End match" lines for a score counter. This could be a counter for each paddle, which increments every time a paddle makes a successful hit. To discriminate between paddles one edge-triggered counter would increment on the positive transition of "Paddle hit" while the other counter would trigger on the negative edge.
However some gating will be required to prevent a miss of the ball which ends a match from registering on the respective paddle counter as a successful hit. On a match-ending miss, "End match" goes high in response to a toggle of "Paddle hit". To form the clock for the score "hit" counter, "Paddle hit" will need to be delayed first and then ANDed with the inverse of "End Match". The delay just needs to be long enough so that the version of "Paddle hit" clocking the score counters cannot change state before "End match" goes high (this could be implemented rather trivially with an RC delay prior to a gate with a purposely defined logic threshold). When the "Serve ball" pushbutton is pressed to start a new match, "End match" goes low. This could clock a negative edge-triggered monostable providing a reset pulse to return the score counters to zero at the commencement of a new match. I think a cool display for the score counter would be seven-segment LED displays made up of discrete 3mm LEDs, say four in a row per segment. The score counters would be two digits each.
This is all rather trivial from a design perspective and I could knock up the schematics for a score counter module as described, if Timb just routes the mentioned lines (along with GND and +15V) to a 4-pin header. This module could be built as an optional design if desired and just connected by plugging into said header. However it won't be a trivial number of additional components to solder - Four 4-bit BCD counters with seven segment decoding and the associated control logic would take at least 80 transistors, not to mention the diodes and passives.....
An on-screen score display would put the entire project in another league of complexity. I know precisely, because I have 100% designed it already - That will actually eventually be an analogue implementation using integrated circuit op-amps, comparators, switches and logic, however, for the sake of my sanity.