Hi Alex Cox,
I understand why you would want to do this and i have seen instances where this was done (not with the shoping aggregator markup but using the product markup with aggregateratings) and they got the rich snippets in the serps, like you mention. But for how long?
The point though is that your not following the intentions of the schema markups. These reviews that you would be using on the category would be from people reviewing your products and not the category itself. Hence you are gaming the system.
I´ve had the urge in the past (had to use some restrain) but refrained from doing it because at the end of the day you are eluding your users and thats not good business in the long run.
Hope my thoughts helped