John T.,
Many multis, including heavies, have a minimum weight (mass, if you wish) included in the CG envelope and this, generally, would be to limit Vmca.
Looking at the Columbia example cited (an aeroplane with which I have no familiarity .. I presume you are considering the following
TCDS) I suspect the limit is more likely due to tail structure strength or, possibly, pitch authority for the landing flare .. rather than load factor,
per se. I am, as usual, open to dissent ...
You'll get little dissent from me since you've identified some of the
certification requirements in addition to Vmca for the multis. And yes, the TCDS you linked does indeed cover the specific model to which I referred. I'll let others comment on the details further.
Cheers,
Tom