Sorry but I don't see any point in cropping FX in camera for "reach". Just like cropping DX in camera.
You spend a lot of money to buy a 40-50Mp camera and then you reduce it 15-20Mp?
Why don't just shoot the full frame (I think this time the term full frame is properly used) and then take the decision in postproduction:
you might find, if nothing more, at least a better composition.
I can't believe you crop in camera to save space on the card. Just buy another card if that's the reason.
You don't get more reach, you're just cropping, that is losing resolution, or better, pixels.
Shooting DX, yet again the full frame of the DX format, on the other end, makes a bit more sense:
at least you're not going to lose pixels.
But must be considered still a crop if you're using an FX lens, in the sense that the image will have the perspective of an FX lens.
Ok, this is irrelevant with tele lenses but might be visible using wide angle lenses.
In my opinion, if you need more "reach", buying DX or CX is a better practice then cropping.