I say just create an /opt tree off the root partition, which will of course be on drive0 anyway. But without user intervention, I think it will get dropped during an upgrade because the entire root partition is re-created during the upgrade. The solution is to use the spare hda2 partition and mount it to /opt, but not everyone knows how to do mkfs and set up the mounts. Someone was working on an alternate root partition type system which would help us out, but I don't think it's come to fruition yet.
I personally don't like putting anything under the /empeg/var tree myself due to the fact that it's where the player's Important Stuff lives and I'm paranoid.