My recollection was from a friend of a friend who worked for Bombardier on the Voyagers. I've just checked
RVAR 1998 and the only regulations relating to one-per-class are wheelchair spaces (15.4), not tiolets. RVAR 1998
does however say that "the nearest toilet cubicle to a wheelchair space or wheelchair-compatible sleeping compartment shall comply with the requirements in regulation 14" (20.1), but that could be addressed by putting the wheelchair space for 3rd Class at the front end of a coach, that for 2nd Class at the rear end, and the accessible toilet between them.