If I had the skills and time to develop a whole new federated platform this is absolutely be what I would work towards. Sadly I have neither.

OsmAnd is a navigation app, it wouldn't be the right place to start.

My best idea for the easiest possible integration would be to allow for OSM IDs to serve as attachments to posts in the Fediverse (such as mastodon). The lowest effort would be to simply attach ID codes as hashtags. An example of a mastodon post could be:

Impressive, but a little tacky. Worth a visit. 9/10 #OSM #43768260

Here a bot would search for posts with the hashtag #OSM, and include posts with valid numerical OSM IDs as a separate hash tag. Scoring could be included in the text, and be flexible (2/5, 7/10, 87/100), with clients reading the reviews optionally converting them into any form of indicator.

The challenges of this approach (that I have thought of) would be:

1. Adding the tags would need to be automatized somehow. Mastodon's share functionality could be used to integrate the functionality directly into apps and maps, but this would not work across the Fediverse. OSM IDs are not generally easy to find, so users posting in the format manually is unrealistic. Ideally apps like CoMaps would allow for Fediverse sign-in directly in the app.
2. Content visibility is a challenge unless some centralized service is tagged. Adding a centralized service would add further problems. A solution would be to make the service dependant on tags.pub for guaranteed hashtag visibility.
3. Moderation is another challenge, especially if there is no centralized authority. Mastodon is doing work towards shared blocklists on the Fediverse which might help in this respect. Reviews from users blocked by major blocklists would be rendered invisible.
4. Reviews viewed from the Fediverse wouldn't automatically link back to the OSM location, leaving Fediverse users with lacking context. Using a form of attachment rather than hashtags could solve this issue, but it would require more development.

The benefits would be that the federated infrastructure doesn't really need to be developed much - we would just need people to agree on the standard and find ways to display reviews.

An app to post reviews making use of specialized content fields would probably be nice, but I fear anything that requires additional accounts or apps would deter people. Being able to sign in to existing Fediverse accounts inside of map apps and posting reviews directly seems like a better solution.

I'm sure there are many people with more knowledge than me on the matter though, and some of them are probably working on this already. As I said, I'm just daydreaming. :)