JC Viggen hacked a BIOS and opened up the multiplier option on the SK8N. Ace's Hardware used it in their review. Apparently the 940-pin Athlons are unlocked, but the motherboard manufacturers are not leaving the option enabled.
You'd have to be pretty good with hex to figure out how to unlock that option. Asus's BIOSes have a proprietary encryption so you can't get at them with modbin.

I think the SK8N BIOS might be a Phoenix BIOS too, so that throws everything in a loop. You'd either have to get your hands on Phoenix's editor or again, be really good at hex.
Hopefully someone will come up with a solution. I did a bit of research and I just don't have the skills to do it. I'd certainly buy an Opteron 140 and an SK8N if I could have multiplier control, though.