You could get a Euro Wii and mod a SCART cable... Should be as simple as boosting the signals to arcade RGB levels (aka normal SCART > JAMMA operation).
I believe it's almost always going to output 480i however, only some applications support 240p (mostly homebrew emulators and NES VC games).
I know for sure SCART has been disabled on USA Wii, you can get 240p from component cables on NTSC but it's YPrPb not RGBs.
*WARNING*
I believe it's possible to turn a NTSC USA Wii into a PAL Euro Wii by forcing a region change in software then flashing that regions firmware (4.3E or 4.3U for example).
It should be noted flashing a new/different system menu is very dangerous, if ANY problems arise a software bricked console will be the result.