I haven’t actually tried a trailer behind the unicycle (I’ve rigged one up to push, see below), but I thought I’d post some thoughts…
Basically there are 4 types of bike trailer attachments that I know (I’ve owned all 4 types):
a) seat post mount (e.g. Tout-Terrain)
b) left side skewer quick-release (e.g. Chariot, Burley, Weber, most high-end kid trailers)
c) left side swingarm (e.g. many cheap kid trailers, I think mine is Kid-Cargo or something like that, but I switched mine out for a Chariot mount)
d) 2-sided skewer quick-release (e.g. BOB).
Although it seems like it would have the best riding characteristics, the seat post mount (a) presents the challenge that it could get in the way when dismounting to the rear. In the picture above in the thread the connection bar is angled sharply downward which seems better than mine. Mine stays at a height just above rear tire, which seems like it would be really difficult/dangerous in a rear dismount - in theory I guess you could kind of tilt the uni back as you dismounted so the connections point would go down but this would require that the mount allows for this movement (as a bike only “rotates” this way when doing a steep hill or a wheelie, not sure if there’s enough range – on my BOB I can do a wheelie, so up to about 45 degree angle does work, which might just be enough). Even so it could be close as doing a ear dismount would require controlling the unicycle by holding the seat and rotating it back, but in a UPD this might not be possible. Here the uni size makes a difference: I would probably most want to pull a trailer with my 36 or a 29 so you’re up high. With a 26 it would probably be more feasible.
All of the single-sided mounts I have had (b & c) have the problem that the connection has some play so “strechtes” when riding with a bike (e.g. when riding a bike while standing and cranking, you push the cranks with at first low load and then the connecter tensions and the load increases a lot and then kind of rocks back and forth). It’s annoying on a bike, but I think on a unicycle it would be really hard to ride with any significant weight.
(d) 2-sided skewer quick-release: I have a really old BOB-trailer and used it a lot with a mtb. In theory I think it would work, you would just have to adapt the attachment that is designed to go on both ends of the quick release skewer of the rear axel. I forgot exactly, but I think the width should have an inner width of 135mm, so is a little wide than a uni. As I don’t think it would work to attach to the crank bolts (as they rotate), the options would seem to be: a) put a clamp like a pipe fitting clamp on each leg of the frame and attache the BOB there, or b) put longer screws on the fork clamps (forgot the name) and then make a custom metal piece onto which the BOB attaches.
Otherwise I would think the BOB would be the best choice with the two advantages: 1) the two-sided mount has almost no play, which should make uni riding much more pleasant, and 2) the mount is low down near the level of the axel so for rear dismount you just have to spread your legs a bit and it should be ok.
I’d be interested to see what you try out. My motivation is a little different as I transport my 2 small kids almost daily in a bike trailer and then have to bike instead of taking the uni. I have set up one of the 2-wheeled trailers (Chariot) with an extra long handle and can then push the trailer (the problem with the normal handle is that the trailer is too close and the wheel rubs on the rear of the trailer, plus with my 36 the handle is way too low, so uncomfortable). With my extra-long handle (extended about 30cm) this works ok, but basically turns the uni into a tricycle so riding is less “fun”. Plus, slow-speed turning and such at lights can be kind of hard although I think with practice it could be mastered.