I can see there's openwrt for Asus RT-N11P/RT-N12+/RT-N12Eb1 and RT-N12 [A-D]1, VPB1 and HP but nothing for plain N12. I think the RT series was successor to the N12. It's old. https://www.asus.com/uk/supportonly/4g-n12/helpdesk_bios/
I'm asking here on the off-chance the RT-N12 variants might be a good enough fir for this
thanks,
Usually not. There is further information here (but the wiki isn't working properly right now, so we can't see it 
). Typically, the hardware revision information like VP B1 mean that there may be specific and material differences between models, unless someone has confirmed that the hardware is actually identical.
Unless/until you have confirmation of compatibility, there is a good chance you will brick your device if you apply the firmware for a different version of the product. If you don't mind that risk, feel free to try, but I personally would hold off and/or do a lot more reserach.
OK thanks. Upgrading the firmware to openwrt on this device doesn't fall under the category of 'must be done', more like 'it'd be nice' as it seems to work as expected under the stock fw.
Looking at https://wikidevi.wi-cat.ru/ASUS_RT-N12 suggests that most (all known plain-) rt-n12 hardware revisions are Broadcom based (so completely different from the aforementioned mt7628 based RT-N12 VP rev. B1), that means wireless support is basically non-existant, nor does either of them meet minimum system requirements (3/32 and 8/32). Combined with the rather ancient, low-specs hardware, it's safe to assume that neither of those will ever be supported.