Every horse that I've worked with has learned to stand in the aisleway before and/or after I ride without being tied. Mostly because I'm lazy, but occasionally because I consider it a good training tool. Can your horse stay put where put if he knows he isn't attached to anything?
This would be the aisle, absent of Tiffany who promptly hit the road the second I unclipped her and moved 6 inches to my trunk. Oooops. Obviously you couldn't put a bridle on her after you slip her halter off while she's tied to the trailer.
Round two. Reinforcing WHOA and moving back to reposition her feet every time she moved them. I make my disapproving noise if she moves. It's hard with Tiffany who is used to being ridden but doesn't have any sort of a bond with people so there's really no reason for her to stand somewhere and just risk getting worked more.
A few minutes later. I'm working on a reward/punishment method. If I have to come and catch her I scold her gently and put her back where she was. If she hasn't moved I give her a cookie. She's very treat motivated so this had good results. Needs more work though.