The params.dat file has options to instruct multichain to use a native currency, and issue a mining reward for each block, although if you configure multichain to allow any party to mine, then beware that someone could use an ASIC to cause havoc on your chain.
It's not possible to issue assets as a block reward, so you would have to make an agent that validates some kind of POW system, and issues the asset when work that meets the difficulty is presented.
