@johnjohnston I think you should be able to do it by using the ?cat=-_NN_ exclusion syntax, with the caveat that, for reasons that are beyond me, it seems exclusions only work for categories, not tags.

So instead of tagging your to-be-crossposted-to-Twitter posts with tag “Twitter” in category “micro”, you file those posts in two categories, “micro” and “Twitter”. Then you send Micro.blog two feeds:

/category/twitter/feed/ (this gets cross-posting enabled)
category/micro/feed/?cat=-NN (where NN is the tag_ID aka category ID of the “Twitter” category; this is all the other “micro” posts that aren’t also “Twitter”)

Why exclusion doesn’t work on tags is beyond me (it doesn’t work for specifying a single tag or for excluding a given tag), since they both use the same parameter series (and, in fact, said parameter is named tag_ID!); maybe this is a bug that has been fixed in a newer version of WordPress than the old one I’m still running?

But either way, you should be able to do what you want, with a little tweaking on your end. // @help @simonwoods @jgmac1106