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yes but the force has to take into account the actual cog in back, otherwise it's meaningless, especially if were talking velocity of the bike.

i honestly don't see the meaning in measuring realtime power before it hits the road.

a force of 5 newtons exerted by the rider at the crank when in the small cog, means a very different thing than when in the big cog. unless the velocity slows down exactly linearly as the mecanical advantage increaces???

In your last paragraph, you are struggling towards the basic understanding of mechanics which will allow to see that measuring at the crank is the best way to determine the effective power output of the rider. For all practical purposes the rear wheel power measurement is fine too, the number will be slightly lower as transmission losses are already gone, and not consistently or linearly related to crank power as transmission loss varies slightly with gearing and condition of the chain.