The current HockeyApp plugin does not provide support for DMG and PKG files. It is possible however to use their API to upload these two file types, it's just a little trickier to do that than it is to upload an IPA or APK file.
My teammate and I have written a script that uses curl to upload DMG and PKG files by using an existing app ID and API token (similar to how the HockeyApp plugin work now) to create a new 'version' for the app and then to 'update' that version with the DMG or PKG file. We know that it's possible, and are currently using a mix between your plugin, which is desired, and our script across the multiple build jobs that are set up for various products so that we can get them over to HockeyApp as a post build action.
Adding support for these two file types would be incredibly beneficial to us, and most likely to several other developers as well. I have attached the script that we wrote so you can get an idea of how this is done using curl.
Thanks for your time.