Update 2: (very) partial success.
As mentioned by @YadaYada here the dv capability is apparently determined by an efuse that we can’t change. We can however, change the code that reads that efuse, which I have done by simply adding the line efuse_mode = 0; before the dv capability is determined by support_info = efuse_mode ? 0 : 1;/*bit0=1 => no efuse*/ in the file amdolby_vision.c.
After doing this I am now able to trigger the DV logo on my TV when using player-led mode - no video plays though and the screen is black. In TV-led mode, I get a pink screen - but the DV mode does not trigger on the TV. Instead it reports BT.2020, but does not report HDR.
I have no idea if any of this info is helpful, but it does show that a non-licensed device can output a signal recognised as DV in player led mode. Next up is trying to do the same in TV-led mode…