Custom vertex streams and shuriken stretched billboard

I've been working on various effects that are dependent on a particle's size, using custom vertex streams and custom shader. The shader corrects for various things based on the size of the particle.

My effects look great when I set the size in the main module, and I can have rectangular billboards.

When I use Stretched Billboards in the renderer module, the new stretched size seems to be ignored when passing this data using custom vertex streams.

Is there perhaps some way that I can access the stretchiness? Alternatively, if there's a way that doesn't use stretched billboards that still allows me to rotate a particle in the direction of its velocity, that would suit my needs too.