Can a bitcoin block reward be paid to a multisig address if the transaction is included in the block? Can bitcoind or other miners be modified to use a fixed payout address?
Sure. The miner decides what the payout transaction looks like. It can be paid to any address using any valid type of transaction.

What's specifically checked by the network on coinbase transactions is the absence of output reference in the input, the coinbase position in the block (first) and the reward amount. The output script can be any standard script.

As a matter of fact, rewards used to be paid as pay-to-pubkey transactions. Nowadays, most pools have transitioned to using pay-to-pubkey-hash transactions.