Finally, I got my MP1,1 installed with Windows 10-64bit. Except for an issue (Windows froze after 10min running youtube video and needed a hard reset - power off and on again.).
1. Got a Windows 10 ISO. (I got version 1803, good for WindowsToGo)
2. on a Windows PC, use Rufus to make a USB installer, to a 80GB HDD installed in a USB box.
3. Removed the HDD and inserted it MP1,1
4. Boot from that HDD (use option key when power-up)
5. Install windows directly on that HDD, no formatting, or else. (This was a glitch, whenever I boot to this HDD, it gives me a menu to either go straight to windows or install windows)
6. Got a Boot Camp 6.0 zip file from Hrutkay Mods (search Youtube for his instruction video and BootCamp files)
7. Run Boot Camp
8. Manually download and install Nvidia 7300gt driver for windows.
9. Reboot the machine to Windows to verify BootCamp Control Panel work. Turn off MP
10. Swap VGA (Radeon HD 6670), turn-on again. This time MP booted to EL Capitan.
11. Reboot to Windows.
12. Windows installed the driver for HD6670 itself while I was trying to install Catalyst (AMD driver installer).
Yes, the Windows on my MP1,1 tends to freeze now and then, but I just need it running long enough to flash bios for the Radeon R9 280x I plan to buy.