Most multiplexer provide a low resistor between the IN/OUT, i don't see any kind of problem by using it. just make sure you smooth the signal at the output and you should be in business.
I already done something like that with almost the same requirement. The main difference was that i used few a/d and few multiplexers and all the POT was always connected to AVdd and AVss, the output to the Multiplexer, multiplexer out to an opamp (to match 100K impedance), then to the PIC a/d.

Have fun!