Just by the way, I've found I don't have to use VPN-PBR for IPv6 at all now (and can't get it working, anyway).
Simply adding the route:
config route6
option interface 'mullvad'
option target '::/0'
and then specifying which prefixes get dished-out on which interfaces is enough to direct traffic over my VPN (thanks to NAT66, of course) or WAN.
Inevitably, my stubborness will probably cause me to try and get it working with PBR at some point; but for now, the outcome has been achieved even if the means weren't as I had planned.