Since the Noctua NF-R8 redux fan worked fine with the beQuiet fan sending signal to the MB for BIOS check, I decided to swap its internals into the original G5 Delta fan. This was a little tedious, but not that difficult, thanks to all documented in the G5 Fan thread.
My G5 upper shelf fan is a double unit, with a standard-style 80mm fan, and a "hamster wheel" style fan next to it. I modified the standard fan, and won't be using the other one, although it would have been interesting to take it apart and see if it could be used. I pulled apart the Noctua, with the only casualty being one of the fan blades cracked where it meets the center - these are fairly thin and the connection won't tolerate much bending, so beware if you tackle this, the Noctua will likely be rendered non-usable. The board and stator were pretty easy to remove, just carefully pulled away after freeing the wires. I did this by hand, so as to not damage the iron or windings.
The G5 Delta fan was a little more challenging. No drilling was needed, as mine had a sticker covering the spindle.
Peeling the sticker away exposes the spindle and plastic c-clip holding the fan in place.
I removed the plastic circlip with a jewelers screwdriver and very thin tweezers. This job was tricky, as you can't damage that tiny plastic ring, as it keeps the fan in place. Below the ring is a sealed bearing, which fell out when I turned the fan over. There is also another sealed bearing on the other end of the tube, supporting the fan shaft, that one stayed in place. Once the plastic circlip is removed, the fan blades can be removed, after removing the wire cover of course.
The stator and board were difficult to pull from the Delta fan, it required quite a bit of twisting/pulling, and ultimately it popped free but I damaged the stator and the wire from the windings connecting to the board broke. Oh well, no loss as I wasn't going to reuse this. Sorry, no pics of that part, but it is exposed when the fan blades are pulled out.
The Noctua stator and board pulled out easily, I was very careful to not damage this. It was a tight fit over the plastic shaft in the Delta fan, but it is therefore held firmly in place. I routed the wiring just as the original was done, and snapped the plastic circlip back over the end of the fan blade shaft. Next step was to plug it in and test, and it is alive! Very quiet, too, as I suspect it is turning at a lower rpm than the original fan since the windings and stator are much smaller in volume. My Dell board doesn't recognize the Noctua in BIOS for startup, so I don't know
the actual running speed, PWM info is from the beQuiet fan on the same header.
The only casualty from the Delta fan was one of the rubber mounts broke apart when removing the fan from the bracket, easy to replace. All else was fine. Now I just need to take the shelf out to mount it...that's a bit of disassembly work for another day. Thanks to @atl285 for his help, as well as the others in the G5 Fans thread for documenting this work.