Success! :-) GREAT SUCCESS! :-)
I have found the way to start the firmware downgrade: I were right when I said a step was missing, I found the answer reading this page on Apple site
https://support.apple.com/kb/DL1175If you look at point 3 of the article it say:
Press and hold in the Power button until you hear a long tone or see the Power button light flashing repeatedly. Release the Power button when you hear the tone or see the Power button light flashing. The update starts automatically.And this is exactly what is needed to do: starting the Mac keeping pressed the power button! :-)
In brief here are the steps to follow:
1) Copy the Firmware file extracted in the way Blitter has said some posts ago, no matter about the OSX installed, a blank initialized partition is good. If the partition is the first in Disk Utility list its number should be 10
2) Shut off the Mac and be prepared to do the following steps as quick as possible
3) Press and keep pressed the front Power Button until it flashes, or until you hear a continuous tone
4) Release the Power Button and be prepared to press the key combination to enter Open Firmware (Cmd + Alt + O + F)
5) When you hear the Boing sound of reboot keep pressed the Open Firmware keys until the prompt comes out
6) At the prompt enter the following line
load hd:xx,Firmwarewhere, of course, the "xx" is the number of the partition where the "Firmware" file is
7) At the next prompt digit
go The screen becomes black, the progress bar appear and the CD tray is ejected
9) When progress bar has reached the end the Mac reboots, the CD tray close and you have an OS9 bootable FW800 PowerMac :-)
The Info panel reports correctly 4.4.8f2 as firmware version and the OS9 boot is available as option in Startup Disk preference panel. I used the OS9General.dmg extracted from the Install disks of my other FW400 MDD.
OS9 boot went fine, I skipped the annoying registration screen pressing CMD + Q and everithing seems ok.
For who is interested in the FW800 port I can confirm it works well under OS9 but I don't know about its speed in this environment if someone has some disk benchmark tool to suggest I will be happy to do some test. In OSX 10.4 the speed is full 80 Mb/s, I have tested it with Xbench and obtained 76 Mb/s with an old 320 Gb Seagate Freeagent powered directly from Firewire 800 port.
I don't know anything about the Bluetooth and Airport Extreme functionality since I haven't these devices.
I will perform further tests in the days to come but until now I am really really happy! :-)
About the firmware upgrade back to 4.6.0f1 I don't know... I accepted the risk of downgrade because this FW800 machine was abandoned in my Mac stuff from long time but if the MDD is a tool of your job I can't reccomend to do this step until you are absolutely sure that none of the functionality is missed or altered.
If Blitter has performed this modification long time ago maybe he can share with us his experience. Here all seems to work very well! :-)
Enjoy!