Ways to multiplex the output of a DAC?
Depending on your need for accuracy it might be cheaper to just buy more dac outputs or use a chip that has many dac outputs on it. More analog electronics will just cost more money. More chips = longer BOM and potentially more points of failure. An analog MUX and a sample and hold circuit, would like have the same cost as a just adding another DAC output.

EDIT:
I just did a search on Digikey:


Cheapest single quantity single channel sample and hold amp $1.65 LF398MX
4 Channel 12 bit buffered I2C DAC is $1.89 (less than $0.50 per channel)
MCP4728T-E/UN


So its easy to see how using a MUX + sample and hold amplifiers can be much more costly than buying more DAC channels. This is true for most situations.