Well currently I think all the zones are just specialized hard coded things. So while it's possible to code anything, the building is probably closer/easier to do for this suggestion. Since you can focus on it with Q and assign jobs potentially with probably less coding. But yes, they have a bit of a fuzzy boundary.
And yes, "building primitive(s)" is correct. Another way of putting it would be suggesting a collection of raw tags that go together. Really I think you only need 2 tags:
1) building tag [FIELD] - specifies a building placed by UMHK in variable sizes like fields, with multiuple reaction sites. Might also need to interpret the colors and tile symbol stuff differently, I guess. This would not be specific to this suggestion. Rather, ANY reaction, even attended ones, could be assigned to custom field buildings. They act just like X(by)Y many individual 1x1 workshops, basically. A couple of tags might not work with it, like [NEEDS_MAGMA] but I don't know if that would even require special programming - it would probably just already correctly not work using existing field layout code, since no field tiles can hover over magma. At the same time, you could also do long term unattended reactions in normal workshops too. It would just tie up the single reaction site until done.
2) reaction tag [UNATTENDED:ticks] - The dwarf brings ingredients and then leaves until # of ticks have expired, then the product is created and can be hauled. Can be applied to either a normal workshop or field.