thanks for the replies - I've done a bit of research into this, and both British Airways and Virgin Atlantic are both very cagey about taking the instrument on board. I booked my ticked with BA, because their website says the following:
"If you have an item of baggage (such as a musical instrument) larger or heavier than the maximum dimensions and weight for unchecked baggage but unsuitable to be carried as checked baggage, we will carry it in the aircraft cabin if:
-you told us before you checked in that you wanted to take it into the aircraft cabin
-we agreed with you before you checked in to carry it in the aircraft cabin and
-you have paid us any extra charge that applies for this service."

On another page it also says the following:
"Musical instruments may be carried in addition to your stated hand baggage allowance, providing the dimensions do not exceed those of a guitar and providing it can be security screened."
HOWEVER, I just spoke to BA, and they told me that none of this was guaranteed, and the whole thing basically depends on what mood the check-in staff are in. And that fills me with fear and dread. Gate-checking sounds like a great idea, but again, I am unsure about how much that proceedure also depends on the mood of the staff at the check-in desk. So my options are:
- buy the SKB case, cross fingers and hope for the best
- check the bass in with rest of luggage in Sadowsky hard case, REALLY cross my fingers and hope for the best
- travel without Sadowsky, buy a cheap used p-bass once I get to NYC and avoid all headaches. (not ideal)