Just a suggestion... compare part numbers from the parts lists for each backhoe. If the subframe part numbers are the same, or cross-reference to each other, it should not make a difference which BH hangs off the back. Hopefully your dealer will take the time to go through the parts list informaton with you... being that you are his customer and all.
If the subframes have matching part numbers, your dealer would have to explain specifically why he thinks they are not compatible. But if there are different subframes, it's a pretty likely something in the backhoe attachment geometry also differs, since Kubota and other manufacturers will try to use common parts whenever possible.
You also may be able to download the parts lists from TractorSmart.com and do your own comparison, if they have data on the BH models you're comparing.