depends on your iopener version. I have a version 1.
i used the version you described to do the mod and it works. I lifted pin 3 of u16, and ran it through a voltage divider. This part i made from a "trimmer" pot (pot = potentiometer aka rheostat aka variable resistor) and a normal resister. The trimmer pot is a variable resistor with a screw in the top, you can turn the screw many times to equal one turn of a normal variable resistor. This means you have very fine control over the voltage you produce. I found my one turns itself off if i crank it all the way to 2.2v but it runs fine at 2.0, i am still running at 200 mhz tho cos i have not managed to salvage the correct size SMT dip switches to add. Anyone in the UK got some spares let me know :)
i cant quite remember the resistor values i used but if you really need to i can post pics of my mod as well as instructions as to how i did it.
i have a big ol' superorb cooler as i plan or really racking the clock speed up one day, you might have better luck with a slimline cooler. Available from your nearest maplins or whatever electronics store is nearest to you. Try to find a k62 plus if you can, i havnt managed yet but i did dig up a 450 the other day. With a plus you can use software to change the clock speed. I have the file for that kicking around somewhere.
btw, good luck with your mod. I'm sure someone will write a difinitive guide to this mod sometime