This is a good idea. It’d be neat to get fediverse services, including lemmy, supporting this. We could make cross site interactions so much easier.
There are a couple share project listed on https://delightful.club/delightful-activitypub-development/
I’m probably missing something, but wouldn’t it be far easier to redirect people to install page of extension for their respective browser? Such extension could then transform the button as needed to point to whichever social web instance.
An extension could work, but some users won’t want to install an extension and other users can’t install them (work computer/mobile browser).
While I think a custom URI protocol would make Mastodon infinitely easier to use (it was implemented but they removed it [1]). Using meta tags would likely run into web security frameworks (CSP/CORS/etc.) which makes implementation rather annoying.
However, for the share API, I think implementing Web Share would be a better solution. An installed PWA can register a share target and you end up being able to share content through the OS native share dialog. This way you can share content through the native Mastodon/Lemmy/Pleroma app on your phone or on desktop.
None of these solutions are of much use until the people responsible for the server software start implementing any of this. I hope this changes soon, but I’m not holding my breath.
[1]: Every time you opened a Mastodon page, the page popped up a protocol handler prompt, and that was annoying. That’s the reason Mastodon removed the feature all together and why they’re extremely hesitant to implement it again.