Thanks for your answer.
It is a bit more complicated, because the flight will not enter controlled airspace at all. TA is 10,000ft, so they can climb fairly high (if want to) in class F airspace.
The question arised, because a pilot-who happen to be a B747 captain while working- was not saisfied with my answer to "Request IFR pickup." I have checked everywhere, but couldn't find anything official regarding the matter.