My thought is that if I could get enough of the right pigments (without the purchases being traceable, which is fairly difficult, alas)

I'd dig a hole somewhere isolate and half fill it with the thermite mix, dump in the body, fill the rest of the hole, insert the igniter and dump the dirt on top.

Then touch of the igniter.

Yeah, if somebody found the site, it'd be suspicious as all get out, but just *try* getting forensics on a chunk of iron with alumina slag on top.

You might be able to prove there'd been a body disposed off (via chemical analysis) but IDing it would be right out.