Chris,
There are not 864x625 pixels in a PAL image. There are (typically) only 720x525 nominal pixels (there are not really pixels per-se since the standard was originally an analog signal and the concept of pixels was not relevant, but that is a different discussion). There ARE 540,000 'ticks' per frame though, but there is not an active pixel for every and all tick period. Note, I am being careful not to say 'clock' here since the TVP5150AM1 requires 2 clocks per 'pixel/tick/demodulated sample' due to 8 bit ITU656 422 output formatting. Other devices can output 16 bit 422 where both luma and chroma are output in a single clock. In this case 'ticks/clocks/samples/pixels' are all the same rate.
Most typical video decoders demodulate a region of about 53us into 720 'pixels' per horizontal line, and (for PAL) 525 lines of the entire 625 vertical lines. Regions outside this 720x525 region are not demodulated and in the time domain the digital output stream does not carry any digitized sample data.
There are some video decoders that demodulate this same 53us region into 960 'pixels' and are often (confusingly) called 960H decoders.
What I think you are looking for is for the decoder to continue to output digitized (but not demodulated) samples during some of the blanking periods. This is what is not supported.
Hope this help.
BR,
Steve