As someone who uses open street maps for statistics/analysis to advocate better bike infrastructure; please contribute to open street maps not google.

You can easily do exactly what you were saying; open up an existing road, add an attribute to it saying bike_friendly: true. Just login (on desktop), go to the part of the map you want to edit, click edit at the top and do the walkthrough. It should look like this

Side rant: The google maps API for analyzing data is so bad it might as well not exist. Even ignoring the painful signup process AUTH tokens, cost of usage, and crappy docs; it doesn't provide access to basically any useful information about roads. Meanwhile open street maps is so easy you don't even need an account; just run a browser command and scrape any data you want, including downloading the entire database.

I might be wrong with this, but I think Google Maps gets some of its data from OpenStreetMap. You might be better off updating the road there and seeing if it transfers over.

It would probably take a while to go through, but even if it doesn't work, you will have added data to an open map that everyone can use :)