Interesting idea...just bought a bunch of NTSCx(2&4) crystals for oscillators to try this...I started out on an Atari 800/Vic 20. (NTSC/2 for Atari, 2xNTSC/7 for Commodore)...but the Atari had the GTIA...the super version of the TIA. If the Arduino were to be clocked at a multiple of the colorburst...I don't see how it would be much of a problem...save for serial communication with a bootloader.