The amount of capacity that would be lost with the required toilets, let alone where those tanks go and what would have to be traded out, etc., for the type of hybrid work the units do, just in my opinion, would not work.
Then again, just adding toilets is the least of the problems that this brings. People do not exactly leave public railway toilet facilities in a great state.
Adding extra batteries with drag capabilities would be more useful in a stranded situation than some toilets that would completely wreck the capacity of which these trains are very packed as they are now, especially in the central operating section.