Rather than copy the basewiki around everywhere, it should be configured to underlay the main srcdir, and pages be rendered from there if not in the srcdir. This would allow upgrades to add/edit pages in the basewiki.
Implementaion will be slightly tricky since currently ikiwiki is hardcoded in many places to look in srcdir for pages. Also, there are possible security attacks in the vein of providing a file ikiwiki would normally skip in the srcdir, and tricking it to processing this file instead of the one from the underlaydir. -- Fixed by scanning srcdir first, then underlaydir, and refusing to add any files from underlaydir if they also exist in the srcdir. However, see security for caveats.