RTV and drill/weld a fitting to drain.
If it is something you plan on changing often (4x4 or boat tow rig that may have water infiltration) I would do that, but really, how often do people actually change their fluid? Most vehicles probably go a lifetime and never have a rear end fluid change.
Also, depending on the vehicle, you may have quite a bit left in the diff with just a drain plug. I know the 9.75 in my truck has at least 1 cup (if not more) of fluid in the bottom of the diff when I take the cover off, a drain plug would leave even more since it would not be at the very bottom of the cover.
I have always used RTV with no issues, just make sure the cover and housing are clean and dry before you start.