Hmm - what sort of future vehicles had you in mind? Way constraints are not designed to work as you imagine here, as they are intended to be non-exclusive (e.g., Underground trains can also run above ground, diesel trains can run on any type of electrified track, narrow canal boats can travel on broad canals). For a situation where X type of vehicle can only run on X type of way, and Y type of vehicle can only run on Y type of way, a waytype rather than a way constraint is more appropriate. To try to replicate this using way constraints, all rail vehicles and ways (other than dual gauge ways) would each have to have one of two permissive constraints (broad or narrow) and not only do I doubt that there would be enough slots left after all the slots taken for canals, this would also be confusing to players by the way in which way constraints are represented in the GUI.
Edit: Incidentally, I think that there is a real economic problem with representing different sorts of narrow gauge using the one waytype: there would be no incentive to build the vehicles from the smaller gauges as the larger vehicles, even if they cost more themselves, would always be more cost efficient. The point of narrow gauge is to have greatly reduced construction costs of the railways themselves, which reduction is in proportion to how narrow that the gauge is. The relative reduction in cost over standard gauge would have to be calibrated to a specific gauge. (If one were to balance the costs of the larger vehicles so as to be less cost effective, then they would end up being uneconomic - in any event, the basic economics of narrow gauge would be fundamentally distorted if vehicles from different gauges were allowed to run on a single type of track with a single construction cost).