I have done posting for some other projects with an 80/20 split in favor of the author. Even 20% is a bonus for the program, so consider that.
Also, I have an app which may be off interest, where authors can login with twitter account and submit posts, you as an editor can chose to edit and/or publish with the posting key (keychain) at your convenience, therefore not sharing any keys.
https://crypto-dreamr.com/Auth
The beta version is live right now if you are interested in helping us test it.