I haven't done this on the G4 iMac but I have done it a number of times on the MDD. It's not a hard thing. For removal, I use two soldering irons to heat up both ends of the resistor at the same time. It lets me do it quickly with no risk to the traces.

For adding a resistor, I use a miniature needle nose pliers to hold it in place while I solder the first end. It's pretty easy really.

Instead of adding resistors you could just use one of those circuitboard trace pens.