It could work exactly like pastures.
slight derail, but I just noticed this, which I'll take out of context in asking that as well as the pasture-menu indicating which creatures are tethered (and caged), can the tether-assignation menu indicate creatures that are pastured?
Pre-pastures, I used to use ropes of different colours to grab a so far untethered/caged creature of the type I was looking for (indicated by having no cage/tether assignation). When it was at the rope, I could check its qualities (gender, relative size, etc) and then go to the red "to be bitchered" tethers and assign the one I'd just examine. Or go to the green "to breed from" and do likewise. Or go to the quantum cage and drag in the not-quite-dregs that fell between the two camps.
With pastures, I can tell in the pasture menu if it's already pastured (though not in which one), or caged (as with ropes, colour coding can help, but I rarely control the cage colour like I do the tethers), or tethered. But the tether (and perhaps the cage, but I don't use them much, since pastures) can only see tether and cage assignations (or potential ones, if one the way there).
I've also a strange feeling, though not yet fully tested, that a creature can be assigned to both a rope and a pasture and end up being dragged between them.
Anyway, not really part of the subject at hand, but for some reason I thought I'd say something here anyway.