I see noone else has responded, I'm interested as well and have just started reading round the topic. As far as standards go, the DIN SPEC 70121 seems to be a legacy EU standard that is implemented by many EVs, and ISO 15118 is the newer international standrd, that includes support for DIN SPEC 70121. Of relevance to OpenEVSE, there are two github repositories with Open implementations of ISO 15118 I've spotted so far that implement ISO 15118 .. JOSEV (Joint Open Joint Operating System for EV chargers) is a reference implementation for the standard, in otherwords it guarantees to implements the whole standard, and OpenV2G.
I haven't attempted to work out how either of these might relate to the software base of OpenEVSE, nor compared the two V2G solutions, or assesed what else is needed to fully implement a solution.
Interesting read: https://forums.ni.com/t5/PXI/Hardware-Needed-to-Capture-and-Decode-ISO-15118-Traffic/td-p/3882935
Without off-the-shelf hardware it will be very challenging, and implementing the full stack might require something beefier than the ESP32 currently used.
I've just read that my new Kia EV6 will suport Veichle to Grid. Are there some standards around this?.