Since somebody here knows everything anyone ever needs to know, can anyone steer me toward a conversion of the factory issue M7 floorplate to a detachable box magazine, similar to the conversions available for the Remington 700 models, which are commonly available? An hour's web search reveals nada, but perhaps I'm looking in the wrong places or ways...

I only have info on the Rem. 700.
You will be shocked if you see the price.
Take a look here:
eGun

If the price for the 700 mag. doesn't scare you away, send a mail to this guy ...