Do you think they would even be able to mount on a 6 inch rim?
Sorry, I can't really answer that. Would it mount, probably; will it mount without causing too much pinching of the tread, can't say.
This whole thing got me to thinking; are we making too much out of the front to rear ratio. I got out my parts manual for my B7300. They show two different rear turf tires for the one front. You could get the 21X8-10 with either a 29X12.5-15 or a 31X15.5-15.