You're asking only during game reset, correct? I'm guessing only in a situation where the game has tilted could this occur, since normally you'd play through all the Wows.

Add a motor switch that pulses the credit unit stepdown if not at zero. Since it will be pulsing the credit unit decrement directly, you need to have larger contacts. You will want to tie this into the reset relay to keep power to the relay until it has reached zero. The easiest way to do this is to add a relay. Credit unit reset relay. That only needs three switches - one functions as a hold and releases once credit unit is zero. Needs to be on a cam with multiple fast bumps, not a slow dwell/rise.

Second switch allows/prevents motor from decrementing Wows.

Third switch releases main reset relay if it's the only thing holding it up from being reset.

Switch on the credit unit senses zero.

Reset Relay->CURR->Motor->Credit unit stepdown.

I can, will, and have been wrong before, but I think that'd work for the stated purpose?

You could also switch it around and have the motor switch activate the credit unit reset relay and that could have an additional switch to pulse the credit unit stepdown. Really lots of ways to attack the same problem.

Add a switch (and a spacer or two) to any of the switch stacks that is on a cam with multiple short bumps. Doesn't matter which one - you'll need slightly longer machine screws to hold the additional little bit of hardware. Route the wires nicely, and remember to use a plug if routing into the head. Use a little piece of heat shrink to ensure the bottom tab of the added motor switch doesn't touch anything.

OK, I see. Well, there is an added ball relay, you'd really want to tie credit unit step-up to that relay vs. to the ball count unit as it moves up and down throughout the game. You'd have to otherwise add a suppression switch during game start to prevent the first five balls from adding to the unit.

The method(s) I've suggested are fully reversible, but...

And above all else... document! Make sure you modify the schematic that lives with the game so that if/when you sell it the next person won't have to chase wire all over the game to understand what you've done.

Not required, but easier to reverse, and see at a glance is incorrect.

Those Williams mechs would work well.