Customize a "category" look and feel?

I have searched for this all over the web and found that I "can" do some theme/template customizations based on category, however in my testing I think it assumes that I'm not actually calling a completely new theme for that category so I thought I'd throw this out to youall and get some feedback. Here is the goal:
I want to have a blog site that is myblog.com/mysubblog where my subblog will be a targeted subject such as pets - so myblog.com and subblog will have 2 unique themes but both be managed via my MU interface - now, I want to give a main category to a client that will be my "expert" on that category - as a side benefit they will be able to promote their website via their "expert blogging" - therefore, I would like them to be able to "brand" their category - with a look and feel that compliments their existing website - is this possible? Am I looney to even think this way?

Alternatively, I would be happy if it was possible to set up a blog at myblog.com/subblog/subsubblog - so far I've had no luck doing that in MU - am I just missing the boat?

Harmony