DGPS module not handling RTCM packets

Hoping someone here knows more about what DGPS is doing under the hood. I’m using a Pixhawk Cube Black 2.1 with Here+ GPS and a Raspberry Pi as the companion computer. I’d like to connect it to the Piksi base station we use for our ground robots’ RTK corrections. Piksi runs a TCP server handing out RTCMv3 corrections. With a little fiddling I’ve been able to connect to the Piksi and forward that port to UDP: which is the port that DGPS uses.

I’ve confirmed that data is making its way into Mavproxy because if I switch the Piksi over to serving SBP correction data, MavProxy immediately starts complaining that the DGPS packets are too big. But it seems like it’s not doing anything with the RTCM packets that it receives. There’s no Mavlink message that I can see in QGroundControl or Mission Planner and the GPS doesn’t go into RTK status.

I suspect my problem is that MavProxy isn’t handling the packets correctly because if it was sending bad data to the GPS I’d be able to see the Mavlink messages at least.