I can't imagine that providing electrical power for a set of secondary door locks would require more than a hefty battery in the van area of the BSO - in fact I wonder idly if you could fit them in the existing battery boxes!I was referring more to the cost of having to drag a diesel loco or generator car around to provide power, plus associated extra fuel/staff costs than the outlay of fitting CDL itself.
And a CDL system would be operated by the guard, obviating the need for safety-critical stewards.