I also have seen it much more out of the UK and around the ITCZ, frequently the E / W descent in / out of Japan seems to produce a lot of St Elmos. The problem is, you could fly 100 times in the area and never get it or fly 5 trips and see it 5 times! Like most things in nature, this will require many hours of nothing video and then all of a sudden it will be there. Problem also is that you would have to get cockpit access the whole time, probably not possible in these times. I suggest getting the help fom an airline and getting them to carry your camera on a certain route for a few months and see if they are willing to shoot it for you if they will?
Rgds