Mark writes, At this point we're thinking of flying in to Frankfurt, then train to either St Goar, or Bacharach for a couple of nights pre Trier.
I like LUX airport - it's small and things go quickly there. The bus ride to the Luxembourg train station is simple and inexpensive - and so is the train ride from there to Trier (about 50 minutes.) If the price is right, I wouldn't hesitate to use LUX for getting to Trier.
From FRA, sure, you can catch a train to Trier, but it's going to cost you three hours +. That's a lot of time on top of a long overnight transatlantic flight!
That said... Your tour doesn't include the Middle Rhine Valley, and if you have time for that (like Mark from Orondo does) prior to your tour, FRA would be a much better choice - it's only about one hour by train from FRA to St. Goar, an ideal base town for a 2-3-night stay.