...If you assign a processing parameter like Adobe RGB in the camera, is this overridden by your raw converter? ...
You are correct! The space set in your camera is not used by converters like Adobe Camera Raw, Capture One or Bibble.
... In the section 'Prophoto RGB in use' Michael talks about tagging the output file with the camera's profile (in C1 pro), I wasn't sure if this meant the assigned profile (Adobe RGB for example) or a generic Rebel profile? ...
In Capture One you will be using your "Generic Rebel Profile" or a custom profile like those from Etc... When you open the file in Photoshop - depending on your color settings - you will be asked to convert it to ProPhoto (provided ProPhoto is your working color space).
In Adobe Camera Raw, you only have the choice of sRGB, ColorMatch, AdobeRGB and ProPhoto (maybe some others, my memory sometimes fails) as output profiles.
Not sure if I have addressed your questions, though?