Hey Mike,
You could have linked this so that people know what you refer to. I did it for you now.
Now, here you clearly see that this Jay Bates has the Z assembly on the Onefinity CNC on the middle position. You would need no special bit if you simply drill your holes into the table with the Z assembly fixed to the lower position once, and do tramming later when you selected the definitive position suitable for your work. I would ask the vertical clamping people here in which of the three positions they have their Z assembly fixed.
Also a bit extender would surely add much more length than you would need. It could be possible that it’s too long then and that you will have to put the Z assembly into the highest position to even use it.
Regarding your future work with vertical clamped workpieces: Again, all people here cut tenons with vertical clamping with their Z assembly mounted to one of the available positions. They don’t need a bit extender. Why do you think you need one?
By the way, should you really want to mill below the machine bed height regularly, you could buy the new Z-20 Z assembly. It was designed for the higher Y-50 rails of the Foreman machine since the Z-16 Z assembly did not reach low enough. With the Z-20 assembly, you could have your lowest Z travel point even lower than with the traditional Z-16 Z assembly, and this would be with stability.