Frass from detritus-feeding beetles is pretty much just lignin and other materials that they are incapable of digesting, so I highly doubt you'd be able to feed them with that. The digestion systems of vertebrates are really quite inefficient and lots of poorly-digested bits and pieces of food are left behind. There's quite a lot of cellulose, some fats, and very tough proteins in feces as well as an abundance of bacteria that can also be nutrients for things capable of digesting them. If it doesn't stink, attract detrivores, or decompose very quickly in moist environments, it's probably low in readily accessible nutrients!