Diodes are to change ac to dc in either a half wave or full wave bridge. And diodes are used in dc circuits , a single diode in a ac circuit shouldn’t do anything.

Do you have the schematic? I’d look at energizing the 1000 point relay through the special relay instead. Need the schematic to see how to do it.

Without knowing what it’s called, look for a rollover special when lit relay. This relay might have more than 1 make/break switch on it. With a small bit of paper, slip paper in the normally closed part of the switch. Rollovers will no longer work when you find the right switch. The normally open part of that switch connects to the replay add coil. Unsolder the replay add wire, tape and set aside. The unsoldered lug is your jumper connection spot and the other end will connect to the 1000 point score reel coil.
This jumper should not cause any interference.
Replays will not be added during scoring because a switch on the 100 point relay is not closed.

If you want it to score 1000, you should jump to the 1000 point relay instead of the 1000 point score reel coil. That will ensure that you get all the proper byproducts of scoring 1000. And if you don't want it to score a credit, disconnect it from the Add Credit Unit coil (or from the Add Credit relay, if there is such a relay.) By the way, is there a relay devoted to awarding a Special? If so, just make that relay do what you want it to do.

As pinballdaveh points out, this game uses AC. If you try to do anything with diodes, you'll just keep digging a deeper hole. When, if ever, do you want to add credits?
.................David Marston

Instead of changing the game’s wiring, maybe reconsider the value of credits. Because what is winning 1000 points—when you think about it practically? A stepper increases by one, and you hear a chime. What happens when you win a special? A stepper goes up by one, but you hear the knocker.

Think of the the credit unit as a counter for how many times you “beat” the game according to the rules designed by its maker. They’re a completely different kind of “point”, but one that is arguably the most valuable.

If the machine doesn’t have a shot for 1000 points, it probably doesn’t have a 1000 point relay. All 1000’s scores are carryovers.

OK, this is moving in the right direction, which I define as "the way the designers would have done it back then." Consider adding another relay for the Special award. One of the switches could activate the knocker. Another could step up whatever stepper or score reel you prefer.
.................David Marston

I've done something similar on a couple of my Williams single player games where the only special was a credit, It does make the winning of a special seem more important and more exciting in a home environment.

Here's the best way to do it if you want it to knock only when winning a special.

You need a relay with a single normally open switch. Connect the coil on this relay directly to the coil on the Credit Unit Step-up. So a credit will now fire this new relay. Now connect one wire of the normally open switch to your 1000 point score reel, then connect the other side of the switch to the same lug of the credit unit coil where you originally attached your jumper wire.

This allows the connection from your 1000 point coil to the credit unit to remain open most of the time, blocking the credit unit from firing with every 1000 scored. When a credit is added by winning a special, the new switch closes along with the credit coil which completes the circuit, pulses the 1000 reel and activates the knocker.

There's probably no room to attach the relay on the back of the backboard. You can just mount it on the inside edge or inside top of the backbox.