To be viable, such an assessment needs to be completely independent, totally objective and carried out by a "recognised" independent body. As Hamrah says, once you have it in place you then need the airlines to accept it, and that is IMHO the major stumbling block; many airlines are very intransigent on operational matters, and trust only their own opinions and interpretations (with the latter think Ops. manuals, and the penchant certain operators have to re-write those books!)
Difficult to achieve; perhaps it is a use for the PPRuNe fund. However, I would beware of waving certificates about, there is very little that can replace the performance on-the-day, be it in interview or the simulator. OK, it's a burning hoop to jump through, but they appear every 6 months in the actual job as well........
What it would give you is exposure, experience and confidence, and that can be advantage enough.
£6