I can speak with some knowledge of the subject.
The nVidia Fcode Rom has a GPU check in the FCode, and will abort if it finds that the card has a GPU that differs from the one the FCode is checking for. Editing the Device ID is not enough, you must remove the GPU check from the Fcode or Open Firmware will not continue to execute the code and you'll never get display from the card.
The Geforce 6200 Rom has this check removed, as it was made from the OEM 6600 Rom.
Arti Itra is really the FCode Guru, and knew just about anything you would ever want to know about nVidia FCode Rom's. He made Roms for the other FX5xxx cards as well as some of the 4xxx cards.
It can be done, any Geforce or Quadro baseed on 2/3/4/5/6 can be flashed to Mac cards, if you know how to edit the ROM. Drivers for Mac OS 9 are another matter. We never really cared about OS 9 to test what works and what doesn't, people wanted to use quartz extreme and core image.
The nVstrap at 0x58 is a fickle bitch, it is an And/Or mask of the resister positions of the card. You are mostly safe if you copy the entire string, with the checksum byte from the PC ROM into the Mac ROM you want to flash, but sometimes things go wrong and the card pulls the disappearing act and will no longer boot in a PC to be reflashed, nor will it show up in a Mac.
You have to lift the ground pin from the EEPROM, and connect a switch, or two wires to it, put it in a PC, boot up, with the ground disconnected and once booted, make the connection. Then you can flash the card back to a good rom.
Knowing how to edit the nVstrap is key, and remembering to give it a valid Checksum byte.
I found with some cards I had to set the nVstrap to ignore the position of the resisters on the graphics card that determine the amount of VRam and use the strap to override it.
You can also append the last half byte of the Device ID in the strap, but if you pick a ID that isn't really valid, then you'll likely be lifting the ground pin the the EEPOM.
Most Geforce cards can be appended to Quadro Device ID's via the nVstrap, and likewise a Quadro card can be appended to a Geforce device ID. Tho if you append the Device ID, you'll have to edit the Device ID in the Rom too, or Open Firmware will find a mismatch and never execute the Fcode ROM. That's not the same as the GPU check, the Open Firmware to PCI Bus bindings state that if an Fcode rom's Device ID( in the PCI header ) doesn't match the ID of the Card, it will not execute the Option ROM.
OS X has support for almost a Quadro cards up to the point that they quit making PPC OS X, but no Quadro features were ever enabled, other than the physical stereo 3D mini din , so the cards will not preform any better than a Geforce Card at the same clock speed.
The Geforce 5200 was the first nVidia chip that had a TMDS in the GPU, but it had weak signal, so driving high res dispalys from the 5200's internal TMDS can be problematic. The Apple OEM Geforce 5200 Ultra used two external TMDS's to drive each display. Some reference design PC 5200 Ultra's had two external TMDS's.
With cards older than the 5200, mostly all use external TMDS so it's a safe bet that if the card has two digital outputs, they can both be made to work correctly with the Mac OS.
Other cards from the FX5xxx family may also suffer from the weak internal TMDS, so it you want to flash one, try and make sure it has two external TMDS's if you want to drive two digital display. With the 5700 Arti found the card didn't preform as it should, was very slow, so no rom was ever published.
I never had any trouble driving digital displays, even Dual Link with 6600/7800.
There is also a monitor definitions section in the FCode ROM, so if your display outputs are not the same as the card the Fcode rom was made for, you'll likely need to edit that if display don't work as expected.
