I'm with Steve on this. Peat moss works great for high humidity setups, I use it with my Rainbow Boas and it works fantastic.

The problem is if kept dry it is a very fine dust that can cause problems with accumulation in the sinus cavities and possibly lungs, but if kept moist could also cause these snakes some problems due to excess humidity.

I won't say you can't make it work, but it won't be an ideal situation that could turn bad quickly. I'd use natural soil before peat moss even.

I would go back to Aspen. I keep & feed all my garters on Aspen & wood chip substrates.