Thanks very much for your replies!
Hard to see how it would damage it, shouldn't be too hard to use a low current DC supply to test off-load.
Might be useful to take a look at the circuitry to see if there any obvious potential issues.
OK interesting thank you! Will using a low current DC supply get much data? The OBC minimum configurable draw is 6 A, so it would just overload a lesser supply very quickly wouldn't it?
For the circuitry, would a photo on its own be any use or would I need to go and reverse engineer the actual circuit? I can probably manage to do that but if it isn't a massive risk perhaps I'll just try it and see.
Perhaps just use the DC charge mode if there is one? It's often called "fast charging" but it most likely will also work at somewhat slower rates. https://openinverter.org/forum/viewforum.php?f=17
There is indeed a CCS fast charge mode in my EV, and that was my initial plan. I couldn't find any EVSEs under AU$50K that support DC charging on the CCS pins — they are all designed for fast charging. So I have to build one myself.
The first problem with this mode is that unlike AC charging which is controlled by a simple 1 kHz PWM signal, the DC charging uses a nightmare combination starting with powerline communications protocols over the same PWM control line and getting worse from there: (ROBO) OFDM, Ethernet, "Sounding packets", IP, TCP, TLS, HTTP and the cherry on top: XML! It's disgusting, and you need a beefy microcontroller to run it, but there are modules available and I am trying to order one (fingers crossed *).
The bigger hurdle with this approach though is that the battery is directly connected to the DC pins in the socket, and it's up to the EVSE to generate the voltage required to charge it, so that requires a DC-DC converter of similar complexity to the inverter that I'm trying to eliminate. It's still more efficient overall so worth doing, and I have a couple of these on order too.
So yeah, if I can reuse the hardware that I'm already carrying around, that would be cheapest, easiest and likely most efficient solution. I wish I'd realised this before spending so long going down the HVDC and CCS rabbit holes. Also, thanks very much for the openinverter link! Heaps of great info on that forum!
*: Probably easier than doing CCS is to just put a T junction on the HV lines from the battery and drive the contactors directly... if only the connectors didn't cost $200+