"Now carry it"
As far as I know the "-o" is always referential. In the subjunctive I think the translation is correct, it is kept as short as possible, the "it" part is implied. In the polite subjunctive it could be "mkibebe sasa", where "ki" is the "it" part.
In addition, even if you do not use the polite subjunctive, when the imperative has an object, it looks like the subjunctive, with the final -a changing to -e. Examples: mpige (hit him!), yabebe (carry it [maji]!); ibebe (carry it [meza]!). That's why I think any answer in this form, where someone chooses to express the object, should be accepted.