I desperately need a Tweetbot for ActivityPub.

No, not for mastodon. I don’t use mastodon, and mastodon does not implement ActivityPub Client, only (mostly) ActivityPub Server. I’ll write my own ActivityPub-supporting microblog before I install a mastodon instance.

@gaelicWizard do you know how (or if it's possible) to reply as username[at]micro.blog to a Fediverse post from a Micropub client?

@kandr3s you need a custom domain set up here on M.B. For example, someone on mastodon can mention me @john because I put gaelicWizard.net as my custom domain and M.B will show it. You can’t do it with at-micro-dot-blog.

@gaelicWizard @kandr3s Oops! I removed my original reply by mistake. Here it goes again:

No custom domain is needed, but you do need to activate ActivityPub.

@sod thank you Sven! I think my AcitivityPub profile is properly setup, but I cross-post from my website that's hosted elsewhere. So replying to posts on the Fediverse isn't as straight forward as from the M.b timeline or apps. I will continue testing...