In miniBB there is currently one level of categories (specifically they are not even categories, but "forum supertitles" which provide the simplest way of forums view on the front page), and what do you have in mind is impossible.
First of all, for a "simple" forum it's too difficult structure. I can't even imagine how it would look in the intermediate pages for example. For example, this topic has the browsing links on top:
miniBB Community Forum / How To /
In your case, it would be let's say
miniBB Community Forum / California / San Francisco / Photovoltaic / Batteries
It won't even fit the page in some cases.
My advice would be to pay more attention to the forums content itself, rather than to categorization. Occasional visitors are usually lost in such complicated structures, which is not suitable for discussions at all.
Modification is impossible in that case, because it would mean to rewrite the many core files of the software. It's too difficult, and it's not the purpose of our software actually.