Hello,
just wanted to share what I discovered, a FireGL V3100 can be turned into a Radeon X600 just by moving two resistors on the graphic chip itself

This picure is a standard FireGL core:

You'll see on the bottom right corner there are a few resistors, 3 horizontal ones and then a bit lower another 2 but vertical oriented this time.
The 2nd horizontal resistor needs to be moved to the right, so that it connects the centre and rightmost pad, the 3rd horizontal resistor then needs to be moved to the left so that it connects the leftmost and the centre pad.
Finally the first vertical resistor must be moved so that it connects the two pads on the left.
Leave the first horizontal resistor and the second vertical one as they are

Here is what the final result should look like, please excuse my sorry excuse for smd soldering

I expected some bios love would also be needed but to my surprise with the stock FireGL bios (Dell OEM reference card obviously made by MSI, green pcb, the same as found on vgamuseum.info) the card was detected as a Radeon X600 with the Radeon PCI id.

As it's still running a FireGL bios it's still running FireGL clocks, 398/398 so slightly slower than a true X600 (400/500), a bit of OC should be needed to bring it to the same level. 2 MHz more on the clock is no issue, but I don't know if the memory will do 250MHz as it's Samsung 5ns (200MHz), it'll probably need a small vmem mod.
On another card with 6ns Samsung DDR (166MHz) giving it "only" 2.85v was enough to make it run good at 230Mhz so should be feasible I guess.

Ideally once you find what your card will do then edit the bios so that the clocks stay for good.

Obviously all the usual dislaimers apply so that if you break the card, scald yourself with the iron, your computer becomes live at 380v etc it's not my fault.
Also note that the resistors are very small, 0402 size I'd guess, so some sort of magnification is strongly recommended.