Probably one issue would be the overall width. If the faceplate is to be round so the opening is easy to cut with a circle jig than the diameter would be based on the length of the waveguide plus room for a mounting flange. Looking at my Wedgies, that would end up with the tweeter opening 5/8"-1" wider than the LKG driver openings.
That would probably mean making the baffle wider for the tweeter to fit. Don't know what that would do to the performance. With a rectangular faceplate the baffle width could be kept the same. You couldn't cut the opening with a circle jig but, like I mentioned, you could easily cut it with a pattern bit.
Another issue could be the amount of time, effort and money that would need to go into developing a second waveguide vs the expected number to be sold. The 1-1/2" waveguide will probably work with any design (OB or box) that uses the M165 woofer family and possibly the M130 also while a 3/4" waveguide would probably be limited to designs using the LGK drivers.
Of course this is all pure speculation on my part.
Mike