Okay, I've not been able to practise this technique, let alone make a video, as my trompette broke shortly after posting! I have now replaced it (and made an improved nut), and am getting this technique up to speed.
Until then, this is briefly how it works: In order to compensate for extra distance travelled on a long buzz of equal duration to a series of shorted buzzes, you offset the next buzzes by 1/2 (easier) or 1/4 (tricky!).
So, (numbering the coup de catre positions 1-4), say a long buzz takes 1 and 1/4 turns of the wheel, by the time I'm finished my hand is just past position 2. The easiest option is to start the next trompette pattern from position 3, effectively inverting the pattern. This is the easier version!
The more complex version involves a more controlled long buzz taking up 1 turn of the wheel, and the next trompette pattern is started at position 2. After another long buzz, the next pattern starts at position 3, etc.
See what I mean about more easy to show than describe?
I have managed the more complex version with just the trompette, and I've just about got the simpler version with some tunes. I'm using this technique as an excuse to play entire tunes in 'inverted' position, and I'm hoping that eventually I'll be able to use this offset buzzing as naturally as I can do long buzzes — but that's a way off yet…