The Lezyne helmet mount wouldn’t be much use I don’t think. I tried it on my aero handlebars and I don’t think the strap will stay put with the light on it, even if I put the light under the bars. Might have another fiddle this evening.
I guess another option for you could be to mount something like a Light & Motion Urban hanging under your extensions, just forward of the elbow pads. They have a rubber strap mount, and you can turn the light 90 degrees, so ought to work OK. Trouble with a lot of bodges like that, though, is that you can’t easily adjust the uppy downy angle of the light without fudging some kind of wedgey thing into the mix.