Modulating, to what end? I've never heard of composite being PWM modulated before, and I can't think of any obvious applications... If you're recovering baseband after filtering anyway, it will surely be easier to simply amplify it, or use a more traditional RF modulation if transmission over any distance is required?
Also, NTSC is closer to 6MHz BW, 3.58 is the colorburst frequency which basically separates luma (below) from chroma (above). And after a DSB mixing process (like PWM), you need to consider twice the bandwidth (since the LSB extends down from the PWM carrier by the same BW), which means for a 6MHz BW you need a carrier over 12MHz, and that's with a "brick wall" filter. (Probably, 18 would be tolerable; a filter cutting sharply between 6MHz and 18-6 = 12MHz, with low group delay, will require a high order type though.)
Tim