I am nearing completion of a build for my son for christmas where I have transferred the mobo from an optiplex 7020 into a gaming case.  I did not anticipate how proprietary the boards were but like you I found adapters that should make the pinouts much easier to navigate.  The adapters and cabling should arrive today and a few more on Monday.  
With the parts today however I should be able to power up the board in the new case.
I'll post all the parts used to make the transition once I have tested everthing.

Dell does not publish specs for wiring.  Its always been proprietary and undocumented.  Any wiring diagrams posted on the web are from users, not Dell.

OPTIPLEX POWER BUTTON CABLE 030WGC

I can tell you what it is.

The wire colors have NOTHING TO DO with voltages.POWER BUTTON WIRES

There are 2 wires for the on / off momentary button.

There are 3 wires going to a Tri color LED.

Careful disassembly of the cable makes this easy to figure out.

I am not however going to encourage this or provide instructions on how to do this. Others have indicated that the red and yellow wires are shorted together through the switch.

Not having ALL wires correct will have "ALERT! FRONT I/O CABLE FAILURE"  Orange Black Blue would be the Tri Color LED.  Educated guess says that the black wire is the common connection for the CENTER of the tri color LED.

I am not aware of ANY Dell that has reset switch.

That is incorrect. There are THREE wires for BOOT and two for LED. See, the board looks for TWO shorted pins already! to be joined by button with a third.

there is a sense pin on the 6 pin power pin of te Opti  So, two of the pins need to be shorted already to prevent an error. then when the third pin shunts it thinks ALL IS OK

Maybe, but I'll take their word over Dell's any day.  What kind of company doesn't let you disable meaningless boot "alerts"?  If there was an actual power cable failure, then how did I turn the stupid piece of junk on to get the message?  Who cares if the onboard speaker isn't connected? Doesn't mean I want to continually press F1.  These are terrible machines with no market outside of institutional settings where a school has to be able to order a thousand units at a go.  

Try returning one of their 4320 projectors under warranty, and then tell me this is a company which has it together.  The best thing I can say for them is...they aren't Apple.

(Yes, I realize this is an old thread, but my Dell rage goes back to '97)

Speedstep may not encourage disassembly, but they really should reconsider, because they are quite wrong (though in fairness, Dell is notorious for changing parts in the middle of a production run).  I have a fairly endless supply of decommissioned 790s to test on.

Giraffe got it right, and here is a more specific description. 

The LED is capable of more than one color obviously, when I tested the LED on a 5v source (Arduino) it changed slightly.  I'm no LED expert, but I'm guessing it controls the color through frequency? 

Still trying to figure out how to use this to negate the pointless boot stopping alerts, but maybe this will help someone else.

@cleopete 

"Speedstep may not encourage disassembly"

I @speedstep   I said disassembly would make it easy to figure out and would not encourage or support or provide instruction on how to do this.

390/790 power button pinout is well known now.

https://www.ebay.com/itm/264733401752

5 pin power switch post from years ago

There are kits now to take care of front panel etc using standard case as well as fan cable adapters for standard fans.

This however will not eliminate All errors.  Some of them are not optional like the thermal sensor, audio front panel etc.

You can just solder the back side of the board to get rid of front panel error and power button error as its only 2 pins.  If your lazy like I am you buy the kit to fix this.

https://www.ebay.com/itm/264733401752

 The thermal sensor and fans are not optional however so you will have more errors if you do not use dell fans.

Adapters to other than dell fans do work but the self test will fail if fans don't go as high as 4000 rpm like some dell fans do.

 Pocaton Dell Fan Adapter

I use dell XG27M fans with adapters so that in the future i can re use fans without needing to modify them.