I’m happy to see the response so far. I hope it rises above the threshold for wanting to build it into ThreadReaderApp as a feature.
I’ll note, hopefully for ease of implementation, that a Micropub solution will already allow you to post to WordPress, Drupal, WithKnown, Craft, Jekyll, Kirby, Hugo, Blot, Micro.blog and others.
There is also an open source project called Silo.pub that provides a micropub endpoint for services like Tumblr, WordPress.com, Blogger, and Twitter (among others). Aaron Parecki has a public version I’m sure he wouldn’t mind if you tried.
Other platforms could quickly allow the functionality and so much more by building their own micropub servers, which would be a major boon to the social media space and the open web. 
If you have questions about implementation while building, feel free to pop into the IndieWeb #dev chat (where prior implementers and others) are available for help. (Alternate chat modalities including Slack and IRC are available if you prefer.)