Brakes: I have two tandems--A 1991 mountain tandem that I retrofitted with Avid Disk brakes and a Paketa Mg road tandem with dual pivot dura ace brakes. If I were to do it over again, I would go with a dual pivot front and disk rear. The dual pivots always worked well on high speed descents, but I was always concerned about overheating the rims. The disk brakes work fine, but I experienced some noticeable rotor warp after prolonged descents with high speed breaking. Similar squeeling if the rotors are not clean. Also,packing wheels with exposed disk rotors in an S&S case may be problematic. Some people suggest removal of the rotors, which would take additional time at when packing/unpacking. One of each seems may be a good compromise. I have an Alpha Q fork on the Paketa which I would recommend, but may not be stiff for a disk brake that is heavily applied in an emergency situation. It works great with the dual pivot.
Parts: I am very happy with the performance of my 10 speed Dura Ace, but am sure that Ultegra would be more than adequate. I have Rolf wheels, but would suggest a stiff standard build wheel such as the Velocity Deep V for your travel tandem. The Rolfs are fancy and look sexy, but I would hate to be on an epic trip and find out that I cannot complete the trip because my high tension/low spoke count wheels broke a spoke that could not be serviced on the trip.
I own a Lynskey ti travel single and seven Axiom ti non-coupled single. Ti bikes make great travel bikes for the obvious durability reasons. None the less, I have some small blemishes resulting from transport, despite careful packing with frame padding. I'm sure a painted or carbon frame would be much more noticable. With the Ti, I simply scotchbrite the frame and the blemishes are hardly noticible.
The Seven Ti coupled tandem is a great choice. In fact, my Paketa has significant corosion that requires refinishing after four short years. Very sad but true. I am still considering whether to refinish or sell as is. Maybe I'll end up with a Ti Seven tandem for Xmas.
Love to see pics when you get it built!