As it turned out, I decided to use node.js on Linux on EC2 instead, as I know that works fine.
EDIT: Apparently, all the information needed to do this is now public, it's just not exposed in any tools. You need to create a cspkg[0] file and a cscfg[1] file, then use the Create Deployment endpoint[2] to upload them along with extension information.
[0] http://msdn.microsoft.com/en-us/library/jj151532.aspx [1] http://msdn.microsoft.com/en-us/library/ee758710.aspx [2] http://msdn.microsoft.com/en-us/library/azure/ee460813.aspx