Here's a quick update (from road with bad keyboard, so short):
I did some more research last night, and then ultimately decided to go ahead with it. This morning I called VA DMV (I could still have returned the trailer with a bit of a back-tracky drive if news was bad).
They advised that when I come in to title/register, I need the bill of sale (got it), a US-DOT form HS-7 (which does not need as much info if trailer is over 25 years old score for 1974), and one of six optional U S Customs forms, validated by US Customs.
Okay, called US-DOT. they don't re ally care since it's over 25 years old and not motorized, but of course I can have the form for V A-DMV.
Called customs next. I could have done that part at the border i f I had known to ask. Customs themselves did not care (and no duty owed due to NAFTA), so although the customs agent asked to look in the trailer (and thought it was cute
), she did not bring up the forms at all.
However, I called the closest "port" to my house (happens to be by an airport), and they knew just what I was up to. Said I could come by on my way through with the trailer and they'd fix me up with the proper form and validate it. Again, they didn't care, for themselves - it's just for VA-DMV's sake.
So, we'll see how it goes.
Sounds good so far.