y0da900's suggestion, especially with his experience with the mech and closed bolt ion, is gold here. Great idea!
I think you meant 4,9 and 6,1 (or for us other folks 4.9 & 6.1), not to be a dick, but you know...
Would it be enough force to cock a nelson? You're absolutely right it has a lot to do with the spring (and friction of course). I'm wouldn't bet it's enough for reliable fully functional operation though. But that is just a guess... (always take more forced than I anticipate to cock a new nelson I find, although this is not true for carter guns I've toyed with)
Of course I still vote cups (and you have the right idea regarding their instal), but y0da's idea works either way if you go the cocker whatever-way route. The cups just seem better as you'd be able to easily integrate them into the design, and of course they work the same as any other cocker whatever-way.