That's a really good question.
What I would want to do is push the big red EMERGENCY STOP button and make the project come to a complete stop. I would then like to call the project sponsors and project manager(s) to a meeting and explain why this project have a high risk of failing when it comes to the projects usability aspects.
On the meeting I would beg for an Interaction Designer and an Information Architect to be hired right now. I would also want to give them a week to get a clear picture of all aspects of the project, and have them present the results of their work no later than two weeks away. Then the project would have to restart with the hired usability team as responsible for the User Experience of the entire application, no matter where in the world User Controls are built. Consistency is key.
This is what I want to do.
If the above is not possible due to organizational or political reasons (or your job might be on the line for calling a meeting), then you need to create a cross functional team. Even if you don't have control of what others develop you can allways colaborately decide which style to use. As you mention in your quesation, to get "user controls feel more unified", all the developers need to be unified in the sense of completeing a common goal. Within that goal is a big part the User Experience of the application. You also know that the application you are colaborately building will never be better than the tasks your intended audience of users manage to accomplish, when the application is published. Unify the teams, decide a common style guide, iterate wireframes and mockups together and get all teams to work as one.