You could do that, but you would have to make sure that it didn't interfere with the gears and you would want to mount it about the same height as the factory plug so that you could use it as a "stop filling" gauge.
If you still want a filler plug in the diff cover you could buy an aftermarket cover with one already in it.
I had a slow leak from the diff cover gasket myself this summer. I seem to remember using a breaker bar or a pipe to get the plug free.
spray with pentrating oil then get an extension and hammer and hit the plug a couple times, thats how i got mine out when it was rusted and painted over