I'm not sure whether this would be possible or not!
When we take our motorhome to Europe I use a custom profile which excluded toll roads. Sometime I want to use the 'peaje' and have a similar profile that allows this.
Mostly though a journey might be mostly toll free but with only a small section on toll roads. For example the A75 in France from Beziers to Clermont Ferrand is toll free except for the Millau Viaduct. If the toll free profile is selected then Navigator routes through Millau (which you would probably not want to do in a motorhome!)
This is a very simple example and ignoring the instruction to leave the A75 before the viaduct is a simple option.
On other trips choosing a wholly toll free option can increase the journey length considerably but a small section of 'peaje' can often make a great saving. To achieve this I set a waypoint before and after the peaje section and change the profile manually. Navigator then re-calculates the route and all is well. It is rather tedious though and only really work when there is a second person travelling or else stopping to make the change.
What is the feasibility of 'appending' a profile to each waypoint which will permit a journey to consist of part toll and part toll free?
If you place waypoint on the toll road, it will be used even if you have toll roads disabled (to reach the waypoint has higher priority then the restriction). On the other hand if there are several small roads with toll on/off/on/off it will probably fail to find route.
I tried this yesterday on a planned route to Madrid which uses two small stretches of toll road. When I selected a waypoint on the first stretch worked and did not try to exit to non toll road before the end of the toll section. So far so good!
However there is a second toll section but I only need to access this right at the end, to route through a tunnel rather than a rather 'interesting' road over a mountain. I have added a waypoint on the toll section just after the junction I wish to join at. The calculated route takes the toll road from beginning to end and I have to add a waypoint at beginning of the non toll option to force it back.
I tried several different ways of forcing the route I want including disabling sections of road. Sometimes it worked but as you suggest I also got a message that it failed to find a route.
My motorhome is over 3.5 tonnes and tolls for a vehicle of this class are exorbitantly high in France particularly and so avoiding tolls whenever expedient is important.
I agree that there are workarounds but they do not always behave as expected!