You could make tree view using a tags-like custom column that is told to break on periods (somewhere in the Preferences, probably Look & Feel, I could check when I get back to the computer if only I didn't suspect you'd find it first
)
That would let you see a tree-like view in the Tags browser.
Granted, at a certain point it may not be worth forcing calibre to sort a certain way rather than just finding something that does it to begin with... but it is certainly possible.
