Finally did that homework Andy suggested and contacted a port of entry: Dallas-FW airport, to be exact.
The authorities there said I can bring in a "reasonable amount" of pipe tobacco and that I should use my "common sense" when deciding how much to bring.
Pipe tobacco...reasonable amount....common sense. Yeah, this guy definitely does not smoke a pipe.
He said that as long as it appears to customs that the tobacco is for personal use or to give out as gifts to friends that there will be no problems. Problems arise, he said, when people bring in entire suitcases full of mostly tobacco products with a few clothing items mixed in.
I tried to press him on what constitutes a "reasonable amount" and there does not appear to be any set standard. Which is amazing since there seems to be standard amounts of just about everything else. Makes me wonder how they determine a violation in the first place. I did explain that pipe tobacco tins usually contain about 2 oz per tin, and he said "Well, I suppose 10-15 of these things would be OK" but I'm not confident he knew exactly what he was talking about.
My problem is that I've bought quite a bit of tobacco since moving to Europe a few years ago including some Hu blends I've yet to try. May have some hard decisions to make as to what stays and what goes on this next trip home.