Not sure Deedee, the US is a lot touchier about imports than the EU, and quite rightly with the fact that such differing climates/flora/fauna can mean something perfectly harmless here can be a monster over there and vice versa.
My only helpful advice is, drop DEFRA or Customs and Excise a line, I had a similar enquiry because I wanted to take home saved seed to France and they were remarkably quick about replying (I'm not claiming they are good people but you might as well make them work for your taxes ).
Oooh and, have a look at the T&M site; they have a catalogue for the US so if you can find the seed on the US online catalogue, I should imagine it will be fine to take it. That's if T&M does rhubarb seed