I have (setq org-clock-persist 'history) in my .emacs (and of course
(org-clock-persistence-insinuate)), and the most recently-used tasks are
indeed persistent between Emacs sessions. However, I like to use C-u C-u C-c
C-x C-i to set a default task for general planning, etc. This default task
does not seem to persist between sessions. Is there a way to either make the
default task persistent, or to increase the number of tasks that are kept in
the history?

I apologize if this answer is in the documentation and I have somehow missed
it.