I don't know personally, but, if you go to Kubota.com and "build your tractor", there are 3 mid mount decks listed as options for the BX25D. So it would seem like a mower would fit a BX with a backhoe and loader, unless something changed between the BX**60 and BX**70 series.
However, a backhoe is not listed as an option for the base BX. So it might all be a moot point. As stated above, backhoe subframe is available only when factory installed?
Or, educate me, aren't the tractors basically the same between the base BX and the BX with backhoe?