Yes I know, that is a problem, but someone mentioned that you could have the unit with all the worker abilities but making all worker jobs except irrigation extremely time-consuming, don't know if that will work as intended, but applying that, you could still have the "teraform" AI strategy flagged.
I guess it will have to be tested. I was thinking that if there is some bulldozer type engineer unit out there then that could be used to more quickly do the other worker jobs. Also since the "irrigated land" terrain type is Landmark Forest, it will have to be cleared by workers in order for Farmlands to be constructed on them.