I like it! It clearly conveys that the mtb:scale must be filled in with a precise value, and if renderers support this in the future, it would give an incentive to end-users to contribute.
It’s a lot more logical than using another key (mtb, bicycle) that is meant for some other purposes (legal access tags).
Something like fixme:mtb:scale=yes similar to what Mateus proposed is another option but I fear it can have limitations:
- renderers may not want to render extra special tags
- it’s hard to read (long) and easy to make a typo
- a mapper might miss it or ignore it in the list of available tags attached to a specific path
Does anyone have a better idea?