I know the VDC can be accessed from 64 mode and that people have used this capability for extra RAM storage, but is video output also available?
I thought it was. When you run Novaterm 9.6 on a C128 from C64 mode, it's able to use both the 40 and 80 column C128 displays.
Almost all the 128's extra featues can be used in 64 mode: 80-column video, VDC memory (really just a byproduct of the VDC being accessible), 2MHz mode, and numeric keypad. The only things you can't use is the full 128K RAM and Z80 processor, but it is possible to trick the machine into using RAM bank 1 instead of 0 when switching to 64 mode. The Servant does this when you choose GO64 from its menu.