I want to temporarily swap parts on two bikes for a trip - I currently have a Tiagra 9-speed setup and a bike with Ultegra 6600 double/10 speed. Is it possible to use the triple front shifter from a 9-speed group with the Ultegra group (substituting an Ultegra triple crank and FD that I'd buy) ?
Yes, it will work, but you need to double check shifting range settings — stops — on the front derailleur.
I recommend use second and third positions of shifter for inner and outer rings respectively (not the first and second).
If you will use not only triple front shifter but also a triple front derailleur you must consider that shifting on double crankset with triple derailleur wouldn't be as fine as with double derailleur (because of triple derailleur frame form).
It will work OK. I've used an 8 speed mountain bike derailleur with a 10 speed group on my cross bike. It won't be the crispest, but it will work. Just adjust the limiter screws until it shifts appropriately.