There was plenty of room on my EZ alum to mount my spare horizontally on the yolk behind the post (Trailer Mods 9 of 13). I carry my spare idler hubs separately in my trailer bag because it was so much easier to use the rim holes to fix the tire in place. As you can see there is not a lot of room to "lift" it off a long spindle.
If yours fits, I don't see why you could not mount the tire ,with the hub, so long as you had a (removable) spindle, or large bolt /washer(nyloc) to hold in place to an added cross plate across the bottom of the yolk, set up so it could not rotate.
May be way different on a 25 or your trailer. Just thinking.
Chris