You could also make / always ro and use that 2 gigs for everything... or you could do a tricky thing (i don't know if you can even do it)

split some partitions and mount in aufs some vital directories as /usr and /opt. Of course it means that when you upgrade you have to split upgrade of packages in realroot with one in aufs root.

This one shouldn't be so painful and risky as usually things in /usr are accessed in ro.