Switch to make MCU pin float
An "Analog Switch" (a.k.a Bilateral Switch) would do the job. Examples include the CD4016B.

They are essentially two MOSFETs connected drain to source to allow bidirectional current flow. When turned off, they become essentially a very high resistance to whatever voltage the ADC output is. 

For the CD4016B above, the off-state leakage current is about 1μA which at 1V for example corresponds to around 1MΩ. Whether or not that is enough isolation depends on how much isolation you require for your application. You can probably find "Solid State Relays" with higher isolation, but they will probably be more expensive too.

You also have to make sure the load is large enough that the analogue signal doesn't get distorted too much by the switch. For the CD4016B this is somewhere in the ~10kΩ region.