Defining a route type at the trip level

The current GTFS specification allows for defining a route type ("route_type" in routes.txt) at the route level which applies to every trips in that route.
There are cases however where some trips in a given route are scheduled with an alternate transport mode. A typical example is replacement bus, where train trips are temporarily replaced by bus trips due to construction work on the railways.
From a passenger's perspective, these replacement bus trips belong to the original route.
There is no way to model this with the current GTFS specification.

MBTA came up with a solution for this use case: adding a custom field "trip_route_type" in trips.txt
See:
- https://groups.google.com/g/massdotdevelopers/c/M35VgMBM2qk/m/up6lYUTpCQAJ
- https://github.com/mbta/gtfs-documentation/blob/master/reference/gtfs.md#tripstxt

Would it make sense to standardize this custom field "trip_route_type" in the GTFS specification?