So... I think T119703: Goal: Connect the priorities of the Community Wishlist and other tech priorities with the Wikimedia Hackathon 2016 is basically the answer to this task, based on plenty of assumptions, but perhaps good enough to work on something practical between now and the Wikimedia-Hackathon-2016?

Let me attempt a quick & dry run:

Criteria for a successful Hackathon project? cf. Life of a successful project?

Short term success: resolving, merging, and deploying, with a focus in the Community Wishlist.
Mid term success: resolution of the projects worked at the event, retention of newcomers,

How to make sure end users will benefit from prototypes etc.? Setting/communicate expectations; followup?

Picking from the CW assures proven interest and followup after the event because the same CW will be pushed in future events, outreach projects, Community Tech work...

Help to get projects more visibility/buy-in (already before Hackathon takes place?) / "into production"?

Same thing, CW already provides a framework for this.

What does our broader userbase get out of the hackathon?

Hopefully the new code deployed during the following week, but an expected wave of progress in the following weeks.

Relation of projects to Community Backlog?

Total relation. :)

How does the Hackathon benefit the goals of the Developer Relations team and their target audiences?

Progress in the CW can be seen as a main driver for Developer Relations. We are connecting many activities to this driver.

For simplicity, if you want to merge this task to T119703, I'm fine with this. After Jerusalem we will need to review our tactics for Wikimania anyway.