Terminology, the first app officially supporting x-callback-url is in the store, and developer documentation has been posted by Agile Tortoise (Yes, that’s me…full disclosure). Terminology is a dictionary and thesaurus app, and support lookup of word from external apps, and also two-way communication, allowing your app to request a replacement word.
I’m starting to spread the word about x-callback-url, and hope that I can start to drum up pledges of support from some other developers in the near term. Please contact me if you are interested in adding support to your apps and have questions (info at x-callback-url.com).
Here’s a demo video of Terminology’s x-callback-url support: