I think this is a really great idea. We’ve discussed incorporating collaboration features (for functionality like that offered by Zooniverse) before. And Jack and I, separately, have discussed allowing users to pay professional palaeographers using the platform. But perhaps this is the best of both worlds.
I’m imagining that the collaboration feature could have two separate functions. First, you could invite other, known users to work on the same documents, so that they can edit them, with version history preserved, etc. And then second, you could offer an open invitation for other users to transcribe a given image or document, in return for so many credits, as you specify, like on Fiverr. What do you think?