Controlling multiple inputs without a common ground using semiconductors
It depends how "isolated" you need your solution to be. 

Given the example above, where the switching voltages are unknown, possibly variable, but fairly small ... let's say they are bounded by some analog supply rails, +/-15V or 0/15V.

Then there's a huge range of analog FET switches designed for this purpose, which translate 0/5V logic inputs to on/off states for FETs anywhere within this range. ON resistance may be 5 or 50 ohms (varying by a few percent as the analog voltage varies from rail to rail), off resistange of many megohms up to a gigohm.

Often used for audio switching in hi-fi and broadcast applications

Good enough? 

They were made by Analog Devices, Intersil, Harris, Siliconix and others, no doubt absorbed into other companies... 

You'll find four SPST switches in a package, dual 4:1 multiplexers, DPDT and the other normal range of switch configurations.

Examples : DG508 and many others.

Farnell's part selector will give you some idea what's available in terms of switch configuration, supply voltage, ON resistance etc.