The Apple Studio Display required 24V and up to 1.87A, which I figured the iGo would handle well since it can be configured to output up to 70W and anywhere from 15 to 24V depending on the tip. The only thing left was how to trick the iGo into thinking it had one of the 24V tips plugged in.
Step 1: The IGo Connector
It appears that Pin 3 is the voltage limit, and Pin 4 is the current limit.
Pin 3 can have a resistance of anywhere from 2.5kΩ to infinity (open). 2.5kΩ sets the voltage to 24.5V and open is 15V. Any resistor in between can be chosen to get the desired voltage in that range. My 13.9kΩ tip tells the adapter to put out 16.6V for a Thinkpad laptop.
Pin 4 is a little harder to measure, since current limits require that you actually draw that much current. The tip had 162kΩ in it, which most likely corresponded to an amp or two. I actually found an article on Neripedia about someone else configuring an iGo adapter and he has the resistances listed that he measured from 9 tips that he has. The only discrepancy is that he lists the current limit resistances as voltage limit resistances and visa versa.
Step 2: Making Your Own Config
The current limit was much less of an issue, so I decided to go with a 50kΩ resistor. That aught to give me a high enough current limit to supply 2A without worrying.
Step 3: Constructing Your Custom Connector
Make sure to leave a long enough lead on the ground sticking out so you can connect your power wires to it. You'll also need an additional resistor lead to stick into the V+ output (Pin 2) since no resistors connect to that.
Once you're done with fixing the resistors in place, you can solder on wires or a power jack of your choosing and you're ready to go! Always coat in ample amounts of hot-glue or liberal use of heat shrink; they're only semi-permanent and cover any exposed conductors!
Step 4: Testing It Out
I suppose that if you wanted, you could put a potentiometer on the limit lines and make you're own 15-24V current-limited variable power supply. Should be good for up to 3-4.5 amperes!