I don't think it would be legal if the grass area is not 'designated' as a runway. Some places do that.
I would not do it anyway, unless I had been over every square inch on foot, or in a car, at least.
Small humps, bumps, ditches, soft spots, etc., that do not have to meet runway requirements make it questionable.
IF it is a good landing surface, and IF there is no tower, and IF you are 100% certain of a good landing, then maybe I might take a chance.