From experience with scan-doublers/flicker-fixers and 31khz horizontal scan capable monitors, usually PAL may not work, while NTSC always works, as PAL scan-doubled is closer to 30khz and NTSC scan-doubled is slightly over 31khz.
It all depends on how touchy/sensitive your monitor is to the lowest horizontal frequency supported.
It's usually best to pick a monitor that guarantees 30khz lowest horizontal frequency in order to make sure that PAL will work.
