Hey guys, just wanted to update you that the modified fan harness works. I'm glad to have changed fans to the Silent 9 fan as it runs at full RPM but is quieter than the stock case fan. In deciding which fan to run at full speed it is obvious to let the case fan run at full speed as it is quieter than the air-to-air fan powered heatsink.
I modified a fan extension cable as shown here, of course, using shrink wrap and soldering the connections for maximum longevity and obedience to OCD. For a few bucks I prefer this method of modifying the extension cable so that the fan's wiring won't have to be modified and in the event that the case fan needs to be replaced 10 years from now (heh!) you can simply swap it out without having to do any modifications to the wiring.
Thanks for the suggestions - I think we can consider this lengthy case CLOSED! Hope it can help anyone dealing with a strange HP PUMP_FAN header in the future as well!