I was searching for "coleman" in the equipment forum and found this thread, which nobody replied to so I thought I'd give this a go!
Option 1
Long tweezers, a flashlight, and beer for after you get the d*mned thing out.
Option 2
Disclaimer: I've never done this before, but it just might work if the pump cup is making a good seal. Drain the fount of all fuel and wipe the filler opening really well. Put the pump rod back in, if you can, and unscrew it a little to get the check valve open. Do not place the retaining bit back on though! Now clean the fuel opening again really well and try blowing into the fount really hard to see if you can push the pump cup out enough to where you can grab it.
Note: Make sure your neighbors don't see you attempting Option 2. That would be serious blackmail material.