Chinajohn,
I can't say how many of the total games played are on unevens. You might as well ask for proof of life elsewhere in the universe, while you're at it. There's definitely an answer to either one of those questions, but there's rather obviously no way to arrive at either of those answers.
What I can do is compare the number of Tour Legends who play the weekly UEL 18 to the number of TLs in the 18 hole single play flat weeklies.
This is not an arbitrary comparison, nor is it one that was cherry-picked for favorable results. It is the only one that I can make that has a complete data set. I can't compare other tiers because their numbers are too large to fit on the 250-player leaderboard. But TLs are still small enough in number to allow for the comparison.
In week 18, 57 TLs played in the UEL event on BO5. 133 played the free 18 on Bethpage.
In week 17, 56 TLs played the Uneven 18 on BOF, and 153 played the flat event on the same course.
In week 16, 64 played the UEL on Bethpage and 137 took advantage of a free 18 on Whistler.
So in the past three weeks, turnout for the UEL 18s has been 43, 37, and 47% of the turnout for the equivalent flat event. That's far from being a minute niche. I'd call it a robust minority.