800x600 you could still get within the lock range of the vertical ramp oscillator circuit with some adjusting.

The horizontal however, you would have to almost change out the R/C time constant in the horizontal oscillator circuitry to be able to lock it in. The lock range depends on the particular IC used, but it can only deviate so far in either direction above or below the center frequency of the PLL/Oscillator. It is the job of the hold controls to adjust the locking point, but sometimes this can go "out of range"