Just tried it out, still got issues, just a bit different as described here.
$ pyuavcan publish 32085.uavcan.node.Heartbeat.1.0 '{uptime: 9876, health: 0, mode: 3, vendor_specific_status_code: 5}' --tr='CAN(can.media.socketcan.SocketCANMedia("vcan0",8),13)'
Error: TypeError: Invalid format: cannot update an instance of type 'Health_1_0' from value of type 'int'