I have no doubt that if these were under warranty that HP would void it if they knew what I had in mind. In a production environment I would never think to mess with them. G5's are actually extremely quiet for a server anyways, they were the quietest thing in the clients rack by far.
But none of that is really the point of my post. I have them now and I want to tinker.
So far I've found out the ML350 G5 requires fans of adequate speed be plugged in to fan headers 1 and 3, it doesn't care about anything else. I also determined the pin out for the stock fans:
PWM Sense 12V GND GND
So it's actually a reversed standard 4pin fan connector with the ground line being split for the 5th pin. They actually split the single ground line to two right before it goes in to the connector.
Crimp part number: 08-50-0114
Housing part number: 22-01-3057 (you can get both of these at mouser or newark for example).
There are no settings at all in the BIOS to control the fans. The server will boot up and get past the initial BIOS splash screen and then complain about the fans and then it reboots.
I read that the BCM/SDR/IPMI are all under the ILO controller so I'm looking in to ways to get my fingers into ILO at a low level. It concerns me that during the BIOS post it complains about the fans though, this might ultimietly be controlled by the BIOS and not ILO. This might be a good thing though, it seems like there is a bit more info out there on BIOS hacking.