I would call a 3.9 kickstart an 3.1 ROM with the 3.9 updates applied to it
While applying updates you might as well add in the Boing Bag updates.
So go and 'find' an 3.1 dump on the Net, follow the handy 'how to make a custom 3.9 ROM guide' on AmiBay and you are good to go!
I can boot straight into my 3.9 WB using my ACA, no reboot needed
