I'm sure it's possible one way or another, if it's deemed to be cost-effective is another matter of course. Via Rail Canada refurbished their Budd Company 1950s-vintage stainless steel sleeping cars with showers at some point; they're fairly basic but very effective. The mark 3 sleepers have (or certainly had in their original state) two toilets per car, which seems like overkill given the relatively low density of occupants; I don't suppose it would be impossible to convert one of these.
Showers are probably more of a necessity in Canada given that a journey from Vancouver to Toronto will involve spending three nights on-board though...
The Mk3's still have two toilets per coach at one end. Whether theres enough room to make one of them into some sort of shower (with somewhere to put your clothes while showering) is to my mind rather doubtfull. Plus your going to need to put in a larger header tank for the water.
Nothing much wrong with the 57/6's, their good for a while yet. I quite like driving the things.