hadw
3
Anything like this would be a third party development, as it doesn’t contribute to the contents or accuracy of the map.
My guess is that, as a small subset of an automatic routing application, it is not challenging enough for developers. If they wanted the function, they could probably knock it up with low levels tools and simple programming, but this would result in something that was too techie for the main in the street.
The other thing that might get it done is if someone could come up with a business model for the service, probably based on advertising. There is a risk that it would be too easy, so difficult to create a unique selling point for the site, meaning that there were be lots of competitors, all only getting a small part of the market.
If I wanted to do it for my own use, I would probably map the route as a relation, in JOSM, export the relation, then use some simple scripting to reformat the OSM file into a GPX. It is possible that this last stage is available, ready done, somewhere, or that later versions of JOSM allow relations to be output as GPX (presumably without time stamps). Posting #8 in https://forum.openstreetmap.org/viewtopic.php?id=23912 looks like it names an application that might do this. Most of this thread refers to routes that are officially in OSM, and we don’t want people to upload personal mapping. However, post 8 seems like it might allow offline relations to be converted.
I would note that umap allows the creation of custom maps, but I don’t if you can export them as GPX, and I can’t remember if they are snapped to the OSM features, or just drawn with OSM as a backdrop.