Thanks for the detail @speedstep. Pinout isn't so much of an issue as I'll have to make a custom cable assembly anyway, and the PWM, 12V and GND signals can be pretty easily identified with a multimeter. 

The more challenging part is getting a connector that will fit the header - I think i'll just have to buy a few options and see what fits best. They seem to be using something similar to a GPU fan header rather than the usual 0.1" pitch fan headers.

You cant order it its OEM custom so what you do is cut the wires and make custom breadboard.

You dont need to use a meter because the color of the wire lets you know.

if you arent cutting the cable then you need to buy an extra fan and cut the wires off and use that.  Or remove some insulation and cut and tap into the wire adding heat shrink tubing and a fan socket header with wires.

https://www.amazon.com/Pocaton-Female-Socket-Connector-Cooling/dp/B073XJV5L9/

https://www.amazon.com/CRJ-Voltage-Step-Up-Sleeved-Adapter/dp/B081K8LBH3

Dell does not support pinouts for anything anywhere ever.

Dell fans often use proprietary wiring order for 3 wire or 4 wire fans.

The wire colors tell you what is what.

Blue     = PWM

Yellow = Tach

Red   =  12V

Black =  Ground

Note wire colors on the fan

Dell to standard adapter

Fan Headers are non standard

So a quick update in case anyone else is trying to do something similar.

Looks like the header is a JST PH-4 (https://au.rs-online.com/web/p/pcb-headers/8201434/).

The corresponding connector is a JTS PHR-4 (https://au.rs-online.com/web/p/wire-housings-plugs/8201478/). This of course requires a special crimping tool if you want reliable connections but you can also buy pre-made wire assemblies. (I'm using https://core-electronics.com.au/stemma-cable-150mm-6-long-4-pin-jst-ph-cable-female-female.html)