The esp in your harman SHOULD prevent overfiring IF i'm understand the operation correctly, guessing at the temps but a heat setting of 2 and the esp wants an exhaust temp of 300? and a temp setting of 6 would look for an esp temp of 450?, the exhaust should never exceed 500. This is based on the stickey of "how your harman works". So if the stove works in that fasion i'd think the pellet/coal mix would just require less fuel. As wagz said, the internals may melt, the esp may react with whatever's released be burning coal. From reading here i've learned that the early models of bixby's exhaust pipe were eaten by the corrosive corn gas. The addition of coal could create a similare situation.
It's an intresting idea and would be curious to the outcome.
Any used stove's on craigs for cheap to use as backyard test subjects?
i googled this "burn rice coal in pellet stove"
mixed reviews here, but looks possible
http://nepacrossroads.com/about8767.html