[gvand]

Imho, this approach could work most of the time (sometimes a candidate could have other commitments and not be able to do contract work on the side).

As an employer i think there is nothing better than to be able to verify beforehand if a candidate will be able to do what you actually plan to pay him for. I would just change that "few hours" with few weeks and favor interaction with other employees (that will then be able to assess the candidate).