Can't run multiple instances of image-creator

I have a nightly cron job that creates a number of Image Creator images and saved projects. I've discovered that the builds are primarily CPU-limited (saving projects is slow - bzip is singlethreaded), so I'd like to run a couple builds in parallel to take advantage of extra CPUs on my server. To support my scenario, Image Creator needs at least the following changes:

1) Each MIC instance needs to unmount only the directories mounted by that instance.
2) Some sort of intelligent locking mechanism to share the host's apt archive cache, or separate apt caches for each image creator instance.
3) Separate configurations for each instance. In my case each MIC instance has a unique apt configuration (different sources.list files), a unique image-creator.cfg (different buildroot/target mirrors and partition sizes), and unique fsets.

I realize #3 could be solved by creating a different platform for each of my builds, but that's really ugly.