Hello, I too would like to do something like this. I am using 2.2.1+ (Build: 20120119).
Here is what I have hacked together so far. I created a Category role called "Reviewer". I gave it all of the "view" type capabilities.
This isn't working like I hoped. For one, I still have to add the users to the individual courses, even if I assign them the role for the whole category. I somewhat expected this but, hey, I can dream can't I? At least we can batch job this part.
The other MAJOR problem that I have found is, in order to view all assignment submissions, the user needs mod/assignment:grade set to allow. This means they could grade, as well as view, the assignments. This is obviously not desirable.
I think my next step would is going to be to look into a way to add a new capability, maybe something like mod/assignment:viewall. Then either modify the core as needed or in some form of plugin.
I have only been working with Moodle for about 4 months though, so there may be a beter way. I am very open to suggestions/better ideas.