HT Chain issues

I have a requirement to replace the mechanical switch shown here, that selects one of 18 different HT levels 0-80v from a potential divider chain, with electronic switching controlled by a 3v logic port expander.

Im currently thinking p-channel MOSFETs for the HT switch, but these need biasing from the HT line they are on, which I think would kill the logic gates! So, I also need a driver device that will isolate the 3v logic control (MCP23017) from the HT and the switch.