Then use a dac on the output side, via demultiplexing, only for the pure DC voltages,
I can't see where that gets you. The output voltage will only be on the one demultiplexed output at a time, it will not be latched.
You could put a sample and hold circuit on the output but that will decay over time periods of milliseconds.
and then use digital pots for the ac voltages?
Not sure what you mean by that.