Thank you all again for the replies! The swelling appears to be going down, at least, he looks more symmetrical than he did this morning. Still no pain with the swelling, and Scout is acting totally normal.
@ mls: Our pasture is both icy and mucky right now. I hate it, but the horses are on weather imposed stall rest. I would not be surprised if the problem is related. I do lead him around for about 1/2 hour a couple of times a day to give him some exercise and keep him from going stir crazy, but even with the walks for the last month or so he's had it quite different from his summertime full day turnout plus hour-ish daily ride. I hate to try to walk him much right now, because his hind feet seem so sore, as I noted before (hopefully just post-trim tenderness, knocking on wood).
@ free_sprtd: I've replied to your pm, if you haven't already seen it. Thanks again! No swelling in legs, belly, or face. The only other issue is the lameness as of this morning. I'm definitely keeping a close eye on him. Several of the possibilities make sense as a cause, with my kind of luck its some of everything snowballing...
Thanks again, everyone!