Customised Assessment Permission - Sign off

At the moment we can change assessment authority for an assessor who is still under supervision. However, this means that the assessor cannot assess and sign off on any units of competency. 
In some cases an assessor may be able to sign off on some units, but may only require supervision on a specific unit. 
It would be great if this permission could be at unit level so we can restrict specific units rather than having to place a full restriction on all assessment.