ATtiny/ATmega programmer board - need a 6 x 1:3 Mux solution
As I currently don't have enough rep to make a comment, I will attempt a complete answer.

First, you most likely do not need an analog demux, a digital would be good enough, since all the stuff is running off 5V.

Second, you could, if you wanted, look for a device such as a CPLD. This is a software programmable logic gate array, that has non-volatile memory. Not as complex as an FPGA in general though. This would make your device capable of programming a different device, just using a software upgrade. Simply wire it up to many pins in the socket. (Please don't stone me for revealing this option)

Third, a 74138 device would probably satisfy your needs.