I'm imagining something like
msbuild /t:CodeplexRelease
which would build, then upload the outputs to the specified Codeplex project.
Has anyone done this?
Here's documentation on using the "CodePlex MSBuild Tasks" for creating and uploading files for a new release: http://codeplex.codeplex.com/wikipage?title=CodePlexMSBuildTasks&referringTitle=CodePlexAPI
You might be able to write a task that uses WatiN or Selenium, but as far as I know CodePlex doesn't have an API for such automated tasks.
Setting up a continuous integration server for a CodePlex project
SNAK - The Simple NAnt Acceleration Kit
Getting Continuous Integration Working With CodePlex: Part III
that should get you started.