If the tenon's are centered in the board both left to right and in the thickness, just butt up against a right angle stop and hold it down.
After the first operation, flip it and go again. If starting at the corner and the height is the same, it will reproduce the same thing the second time. You could always have it go lower to go past the center to make up for any onion skin if going exactly to center.
Same thing if you zero at the center of the tenon. You wouldn't need to touch for zero after the first op. As long as you have a place that's stationary for that time. You could make a board with holes to mount to the table at the same place each time. Have your machine make the right angle to butt up against. Attach some clamps or add some t tracks to slide toggle clamps for different stock sizes. Then create gcode for several sizes that you would use normally and run them when you need them. I think that if you were to do it from the center of the tenon, then you could mark it and move your tool to that marking. If you know the distance from the corner of the room angle, then you could zero there and move the position over the measured amount you need to center the tenon and set, let's say, x zero location.
Lots of ways, you just have to decide how fancy to get
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