I could see a community used for spotting. Not sure how the browser extension part would work.
I see people post a sorta self-spot on mastodon all the time. Something that might be nice is a mastodon bot that could read a certain hashtag or if it was @ed so everyone could follow the bot to see who is activating.
For lemmy: it’s mostly link sharing, so I’d mostly see posting about *OTA experiences, pics, questions here in the POTA/SOTA communities.
Mastodon, on the other hand, would just be one way to show something like that since it’s more time related and ephemeral. Bot was my first stab at something like you mean, but I do think there could be something to auto-post with all that data.
And while not on the fediverse: I use hamspot for something like this. It flips the onus of posting a spot to actually listening FOR a spot from people you know around the fediverse.
BUT if we want to let our ideas get REALLY wild with it: I could totally see a federated service for spotting or any *OTA related activity. Different instances handing different kinds of spots. Parks or summits or islands being different items that people can subscribe to, spot from, and activate. Just spit ballin’.