I've looked into this - it certainly is doable, The coils of the contactors in the HV junction box connect directly to the connector on the front of the HVJB, so would be possible to actuate them to connect the battery to the CCS connector pins. (I.e. there is no CANbus or other intelligence inside the HVJB) As far as I can tell, there is no monitoring of the charger-side HV connections ( i.e. the car wouldn't think the contactors have welded) .
I suspect there might be some monitoring of the contactor coils, as there are seperate + and - connections to each contactor coil and none appear to connect directly to ground or +12V. If there is any monitoring, this could be worked around. Unfortunately I think these connect to the BMS in the battery so hard to access to see what the they connect to.
You can buy DC-input inverters that should work - I bought this one to look at :
https://www.aliexpress.com/item/1005001704095436.html?spm=a2g0s.9042311.0.0.48bf4c4dRALLGm
I'm not too impressed with the build quality, and there is no protection to speak of.
I've also sketched out an idea for how an external CCS plug could signal to some added circuitry to control the contactors to turn them on when my "special" plug is inserted.
However the one thing that prevents me moving forward is that the inverter output is this :
That inverter (and another solar unit I looked at) is not galvanically isolated from the input.
And I don't know that if "something" triggers the car's HV isolation test, which will probably lock out the HV system, whether it can be reset without access to the manufacturer's diagnostic tools.
If I was sure that I could recover from this I would definitely investigate further.
As regards doing it without any mods to the car, I know (first hand from someone who has done it) that on at least one car (ID3 or 4, don't recall which but likely both), it is possible for something that pretends to be a CCS charger to initiate a charge session but not supply any current, and draw current from the CCS pins, without any errors or timeout from the car. I would think this is very implementation-dependent, so may or may not be feasible on other cars.