I snuck it into Vanilla already many months ago, but only for skirmish maps and players still can't build farm fields in neutral territory to make the feature fully fleshed out. Best I can do on some skirmish maps is place gaia fields that you can gather from once you capture the Farmstead. I don't see much downside to just enabling building fields in neutral territory in the base game. Someone might exploit it somehow*, but we can always just catch things like that and plug the holes.
*Perhaps place a Field foundation in FoW and when it gets destroyed you know an enemy is there. I don't think that would give much intel though, because it could just as easily be a single scout cav, so the intel could be dubious or misleading at best. So, this edge case isn't worth denying a feature for.