seems like it would be super convenient as the thumb would be in the perfect place to make shifts when your hands are in the right position. Is the reason they are not used this way the fact that the ends are not big enough to accept them? Is there any other reason why it could not or should not be done?

Mmm, I guess most people who use butterfly bars either have brifters or a separate brake thumb shifter combo, as most will switch a straight handlebar to a trekking bar. So they don't really need bar end shifters.

Unless you are mounting the trekking bar exactly parallel to the head set, in the horizontal plane I mean, there should be enough space for bar end shifters.

i dont believe they work do to the inside diameter of the trekking bars. i have read somewhere that they use paul thumbies with them

You'll be better off with either grip shifters or trigger. Thumbies shouls also work. I don't think bar ends will go.

Bar end shifters won't work on trekking bars because trekking bars are smaller diameter then road bars. i mount by bar end shifters on my trekking bars using Paul Components mounts, which makes them more like 1980's shifters or current trigger shifters. i like the old school shifters mounted on the top of the bar, so for me it was very convienent. Paul makes mounts for both road and mtn diameter bars, so you can mount the bar ends on top of either bar

^^^
this ... thumb shifter conversion of the bar end shift lever, minus the bar end pods.

I don't think the ergonomics work as well if you could fit in the center end,
as it does rear facing on the bottom end of a drop bar ..

me , the trekking bars are on my R'off hub bikes, their grip shift is a perfect match.

That is a beautiful set-up