All of the OKSocial diaspora* servers are able to crosspost any status you post there to a Twitter account. To set this up for your account, login to your server and click Settings->Services from the account menu in the top right corner of any page.
To connect between Pleroma and Twitter, OKSocial has an instance of Renato Cerqueira's mastodon-twitter-poster; this service is only available to OKSocial's users at the moment (we'll see how much load it generates before we consider making it more widely available). To set this up for your account, go to xpost.oksocial.net and connect your Pleroma and Twitter accounts.
We have a dedicated Pleroma instance at twitter.oksocial.net which is using the Stork Pleroma bot to mirror Twitter accounts for our users who want to move full-time into the Mastodon-compatible world but still be able to follow folks who are remaining on Twitter; mirrored accounts are all named in the format (TWITTER NAME)@twitter.oksocial.net. While anyone is welcome to subscribe to the mirror accounts, we're only taking requests for accounts to mirror from OKSocial users. After we've automated the process of creating a mirror account, we'll look at making that more widely available as well.
OKSocial users can request an account be added for following by contacting the HQ Pleroma account at email@example.com.