as an update, i did get this sorted out. posting the info in the event it may help someone else.
first off..... its been years and years since i've had schooling in electronics, circuits, etc, etc. i mostly just fumbled around and guessed whilst hoping to not cook something on the board.
ok, so here's what went down on the fan removal process.....
i started out testing the voltage on the headers while the server was running. only two showed voltage with reference to ground. they only had ~6v on them as the fans weren't running full bore at that time. for whatever reason, at this time the speed sensor wires showed no voltage on them.
next i decided to remove a stacked fan assembly so i could break it down, trace wires and see what i was really dealing with. turns out, its wasn't anything special......
just two 3-wire fans.
the wires in the header are as follows.....
[COLOR="Red"]red[/COLOR] = +12v
[COLOR="Blue"]blue[/COLOR] = RPM Sensor
black = ground
[COLOR="Silver"]grey [/COLOR]= ground
[COLOR="Gray"]double grey[/COLOR] = ground
[COLOR="Lime"]green[/COLOR] = RPM Sensor
[COLOR="DarkOrange"]orange[/COLOR] = +12v
i then, took a small 12v battery and put that to the red and black wires of a fan to power it and used my multimeter to see if a voltage was output on the rpm sensor. there was indeed and it showed a ~6v or 7v signal which then led me to the idea of putting some voltage to the rpm sensor wire on the header. i also measured the resistance across the blue and black wires, this gave me about 60 ohms. from that sprung the idea of using a resistor to simulate a fan as well as hopefully give me a correct signal with which to put to the sensor wire (blue and green wires).
as luck would have it i just so happened to have some 1/2 watt 68 ohm resistors among a few scattered bits i had laying around. i initially used just one resistor to replace only one of the 8 fans in the fan assembly. the resistor however was a bit warm to the touch. i could hold it just fine without any fear of getting burned. but just to make sure it wouldn't burn out i used four resistors in series/parallel. i'm not sure if that makes the capacity only 1 watt or 2 watt. either way, it should cope fine.
how the resistor is wired (very crude)
instead of doing this for every header, i decided to try jumping the RPM signal to each header which worked brilliantly.
finished harness....... or whatever you would call it.....
not sure if anything else needs be added. if any questions, just ask. i've prolly forgotten something. like i said, i mostly fumbled my way through this and got lucky.