And that answers my original question-- There is no M-series cab that can accept a backhoe, at least with factory mounting.
Maybe a second reading is in order, my friend. On page 5 of the attachment you'll see the M4022 Mounting Kit, which says it's "For M4011 with LA1153 Loader on M5040, M5140, M6040, M7040 Cab Tractors and with LA1154 Loader on M6060 and M7060 Cab Tractors..."
Has anyone actually spoken with someone with Kubota about all this yet?