A custom bracket could work. Thinking more about your idea, PVC can be heated up with a heat gun, flatten the end drill holes and mount them where the light mounts are now. Using elbows to extend them beyond the trailer, a T out for the raised lights and plate bracket. Of course a separate ground would have to be ran. The uprights would also serve as trailer guides for backing up empty. Not so concerned about the uprights being in the way as that darn metal plate, like you said raised, but still inside the fender. I usually carry two kayaks too so I need to build carriers for those and again thinking PVC pipe flattened a bit, going from one side to the other, over the boat and using the post mounts on the sides of the trailer with pins to take them off when the kayaks are unloaded.
BTW. Last year there was a recall on those trailer lights. They sent a card and I mailed it in, they sent a new light kit. I still haven't swapped them out because I don't haul at night. Everything around here is close enough that I can usually get home before dark. I have the same problem you described with your lights, but you fixed yours? I thought it might have been the wiring or part of the reason for the recall. You might see if your lights were included and can still get another set. Doesn't hurt to have a second in case you bust one. I haven't even opened the box yet. In Georgia you don't have to have lights on a trailer during daylight hours. Still need to fix them just in case we do get stuck having to trailer at night.