Send MESSAGE_INTERVAL command from mavproxy

Is there a way to sent the message_interval command to arduplane via mavproxy? I’ve attempted to use the cmdlong module, but I get an “unknown command cmdlong” error when I try to use it (and yes the module is loaded). It also seems like a roundabout way to do something that should be relatively simple. Any idea on the best way forward?

long SET_MESSAGE_INTERVAL 33 20000

(50Hz for GLOBAL_POSITION_INT)