I have 6 separate zones each with it's own thermostat and would like to have one master control to override any of the other thermostats from calling for heat. I was thinking of conneting all the low voltage thermostat wires through a 24 hour timer that disconnects the thermostat from calling for heat during night time hours (while sleeping). ANy ideas.

You could use a time controlled device to supply power to the boiler itself, or the zone control box. Or install programmable stats. Or better yet, just turn all the stats down at night.