Started pondering getting video or audio from a C64 by using the expansion port & user port as a pair of DACs.  Normally an offboard chip generates raw data & offloads it to a C64 for rendering.  In this case, the commodore would generate the raw data & offload it to an offboard circuit for rendering.  In the minimum, the user port can generate beeps by bit banging some GPIOs or writing RS232 characters.  It could be an extra limited voice in addition to the SID chip.  

The expansion port can expose a byte of mane memory.  You could theoretically generate an NTSC waveform by writing to the memory fast enough & hooking a resistor ladder to the data pins.