That's kinda like an idea that im gonna try to do. I'm going to make a decent sized run that's permanent, then have a opening on one end about 2' off the ground and ha e it roughly 1'w x 18"h...and i'll have a standard door into the run for me. Then I can build a few tractors ( well more of a coop on wheels ) then on the dryer days I will roll one out and over to the coop. They have a item called a barn door jamb latch, it works off a cam design ( kinda like visegrips..how you can adjust the "force"of it) they are pretty cheap. Id use 4 of these to attach the coop to the run ( squeezing them together ). On the coops door opening id use a 2x4 so id have a lip of some sort to line up the two openings. Just one or the many ideas bouncing around in my head.
You could do a portable run (coopless tractor) pretty easy. Just want to add a roost in there and something for shade, or if they and you get caught off guard perhaps something for shade and rain. GL and make sure you lost pics!