I am working with a team that's building a webapp for internal use in an enterprise. It's basically a workflow app at heart, where there's a single "request". Each request flows through various stages, and at each stage, there's a person or role that is responsible for moving the request to the next stage. "Moving" the request to the next stage might involve adding more data, validating things, gathering input from some external source and correlating it to the data in the request, and so on.

The workflow engine has been selected. The UI for the various roles and stakeholders is being designed.

We have a distributed group of stakeholders. I'd like to employ a collaborative design/dev effort, where devs can produce and stand-up mockups or even working prototypes, then solicit feedback on those things.

In a centralized team this could be done via design review meetings, with everyone gathered round a screen projector. That just is not going to work for us.

So what I'd like is an app that can help with this.

Any recommendations on apps or how to choose?