If I really want to do this, regardless of the difficulty, regardless of the cost, unless the detail of that connector is published, I would have to TRACE the wiring by using a combination of visual and using an ohmmeter, and map out the entire cable. U really want to do this and have infinite patience.
Some Optiplexes have an AIT (Air Intake Temeperature) sensor on that board . 4x Dignostic LEDs also live there. It has been done. But it varies from one Dell system to another. That's why no one can really help you. The answer is if you can't figure it out yourself it's best left alone. My advice is to use the Dell cable and front I/O board in your build.