Ya know, you would think something like that would be in the owner's manual!
Minimig Firmware Manual Release version YQ090421, page 11:
It is possible to override stored PAL/NTSC settings while
the FPGA configuration is being uploaded. This takes
place when the DRIVE LED is rapidly blinking after
switching on the power. Press and hold the [F1] key to force the display to
NTSC mode. Press and hold the [F2] key to force PAL mode. Remember
to save these settings to set current video mode as default.The manual is a little bit old but I have no time to update it and nobody else wants to do it. Besides as this thread proves nobody RTFM!