I should look into plasticizing agents and epoxies. I can probably make a cast of any arbitrary shape I like and fill it with a liquid that subsequently solidifies, and use that as a core for the solid element of a hybrid rocket. Immediately coming to mind is a combination of coal and aluminum (maybe with magnesium?) in epoxy. The form would have more than enough energy to be useful when burned with an oxidizer. I am concerned though, as I've been reading about hybrid rockets being amenable to throttling. I can't think of an especially good way to stop-restart an already-running hybrid when the fuel itself is as cantankerous as aluminum and magnesium. Coal, sure.
Maybe it would be effective to have exactly the opposite of the previously mentioned "igniting fluid": carry a small supply of halon that can be flushed into the monster to turn it off, and use varying oxidant supply for throttle control. You'd probably also have to be careful with nozzle design if you want throttle control that's better than "on" and "off". I'm not sure how some of these really ugly fuels (benzene...) were "turned off" and restarted, as they apparently are capable of doing. We have enough trouble putting those sorts of fires out without the presence of an oxidizer.
Oh, wow; actually I think I have the idea for how to do it. Not ready to share it here yet, but I think all the pieces work.