LordNikon ... I've actually corresponded a bit w/ Dr. Caldwell the stomatopodiatrist (lol) ... he hasn't had much luck setting up an artificial substrate that a mantis can burrow in and said substrate will retain the burrow shape. Generally the sand in the wild is reinforced by bits of decaying organic matter and roots and all sorts of stuff which is difficult (but probably not impossible) to replicate in an aquarium.
The main question is, for me, ok you made the substrate that will hold its shape when the mantis burrows ... what now? I'd imagine that the excess of organics and whatever you use to "reinforce" the sand would lead to severe algae problems, for starters. Additionally, a spearer requires room to burrow, so that basically forces you to have a DSB, blech.