I can appreciate the desire to stay on an older firmware too. I'm on 3.1.0 at the moment (that's what it came with). I didn't know about the USB power problems that started with 4.0.Yes I am on 3.1.0 as all the games I have work with it. I have no auto-update worries. HBC works fine on the vWii. My HDD works as the USB port power hasn't been lowered (these issues started in 4.0.0 when people's HDDs suddenly stopped working).
However, I think the chances of any exploit being backported (if indeed earlier versions are exploitable, which isn't always the case) are very low. There are only a very few hackers working on this, and I doubt any of them have a 3.1.0 or earlier box to play with.