I know more than a few very active skiff guys who simply took their trailer tag off the rear of their rig - then mounted it sideways (lengthwise along the fender's trailing end...) on the trailer's fender on the left side to keep it out of the water as much as possible. Don't think that one of them has gotten a ticket for not having a "tag light" - but that's here in Florida... Texas may take a different view...
My problem with trailer tags - as a very high end user (20,000+ miles a year towing...) is that a standard tag bracket on a boat trailer is so exposed to wind going down the road that within a year or two it cracks away and is just gone so you end up having to file a report with your local cops - then go to the nearest tag agency and stand in line for an hour or two to get a replacement... Here in Florida the cost of a replacement is very small - but the hassle isn't....