Mike Whitson ~ "Does the grass formation depend on having single-crystal Si, or
could it be formed in, say, a low-temp CVD poly-Si film deposited on top of my
SiO2?"

Good question. The grass is independent of crystal orientation, so it should
form in poly or α silicon just as well as in single crystal.

Marc Reinig ~ "Isn't Si fairly transparent above 1.5 microns?"

Yes. Darn you. Still worth a try, I suppose... SiOnxy claims absorption out to
2.5 µm and that is using their non-RIE process that produces blunter blades of
grass.