Wow, you really have your work cut out for you on this one. Do you plan on having a fan with this or just going for the heatsink? I don't have any pictures of my card, but my bro-in-law has a digital camera and I can see if I can't get him over here with it tomorrow and I'll take some pics for of the GF4.
I've tried something similar, though not nearly as ambitious. I had an old heatsink from a 900mhz Thunderbird that I tried putting on there when the stock fan of the HS died. The thing was pretty light in comparison to todays heatsinks, but I just couldn't get the thing to stay on. I could get it on, but after a week or so it would come off. I tried thermal epoxy with dabs of superglue in the corners for extra hold, but it just wouldn't stay. I then tried a couple rubber bands for extra hold, that gave me about another week, but they'd eventually break too. While it was on, I was able to oc my Ti4200 to almost 4600 speeds(my card was always kind of bunk in that respect compared to so many other Ti4200's out there). Anyway, I'm not a real serious modder, but if I was, I could have probably hacked the heatsink and found ways to make it work. However, with your situation, I don't know. That heatsink is extremely heavy.
The problem with ziplines that I see, is that to get them all the way around the card and have good positioning on the heatsink, you're going to run into the AGP slot. With rubber bands, I could fit them into the breaks of the AGP connection on the card and they'd squeeze in there, but zip lines won't do that. You'd have to go to the outsides of the AGP slot and when you get out there, I don't know if you could get a good handle on the heatsink.
I'm not really big into nuts and bolts, so I'm unfamiliar with knurled nuts. I have an idea of what that plan could be, but I'm not sure if it's right or not, a little more explanation would help. In any case, you're going to have a problem with the weight of the heatsink bending the card, which is really bad. You're going to need some kind of support for it. Do you have any pci cards or is everything onboard(this would help a lot)?