This thread (whilst interesting and useful) did not answer the primary question of why excess DC PV power was not exported once the battery reached 100% SOC when DVCC is enabled. I have just found the answer in this thread: MPPT Charger stops generating, excess DC PV not being feed-in - #2 by kbb which is the exact same issue. And it turns out that the issue is that Victron requires the battery BMS to set a voltage limit (CVL) not a current limit (CCL) at 100% SOC. My Seplos v3 BMS first turns the CCL down to 10A around 99% SOC, then to 0.
So at least that makes sense now. I will ask Seplos if they are familiar with the Victron spec and see if they can be persuaded to implement it when in Victron mode. But as that thread says, this may prove to be an uphill struggle. Unfortunately the BMS code is not available so I can’t just fix it myself. And neither is the spec Victron expects manufacturers to follow. This does seem a bit perverse - it’s going to be hard enough to get all the BMS suppliers to follow it - making it harder by having a spec that isn’t even published is not going to help. Any interoperability spec needs to be public IMHO.