I tried it, and on a vegan diet (my normal diet) but I never really made it over the bottleneck.
I think the longest I was able to go without oversleeping was 5 days. I kept trying for 3 weeks but every 5ish days of being perfectly on schedule I would invariably sleep through my alarm and have a 4hr nap which sort of ruins everything.
If I was going to do it again, I'd go outside more often and have more activities and food planned ahead. You get to this point where your brain like... shuts down. You just can't think of anything and you stare at the wall for long periods of time. You've gotta have all the routines ready and in place if you want to make it over that big hurdle around the 4-6 day mark. Plan activities ahead of time, write them down on a big schedule, and definitely use a system to keep track of what naps you've done. Then just follow the schedule and use multiple alarm clocks, and hope you have the self-discipline that Steve has developed.

After my 3rd or 4th oversleeping screw-up I gave in and had the most delicious 14 hour sleep of my life.
Good luck! It might also help to give us a play-by-play in this thread. I'm sure some of us would be interested anyway. I may give polyphasic another go when I have a chance.