Thanks for the route suggestions, chasm. That's a lot like what I was thinking of, although I have trouble nailing down specifics in east/central Europe.
Jonathandavid, I hadn't seen those routes before -- excellent. Maybe even if I don't ride them end to end I can use parts of them, especially the second half of the R1! I'll check those out.
Machka, I hadn't thought about availability. Unfortunately, I expect to be free to start in June, which is pretty much peak time. But I'll look into it -- great point.
Anyone else have suggestions for favorite routes in Europe, especially East or Central?
thanks.