In NL signed rerouting is often marked with signs with a big letter. Meaning: for destination x follow signs with a T on it.
This is not the same as the description of the itinerary. It would be usefull to be able to add this in a sepparate attribute like reroutingSign
This system also exists outside NL and it typically works the way that your whole route is indicated by a sequence of signs (decision points). To that end, it would actually be very similar to a DATEX II itinerary plus a new attribute, as described in the original request.
New attributed added - TG agreed it should be called alternativeRouteIdentifier which is consistent with the alternativeRoute role name.