Is this a single story or multilevel home? Is it average or very tight construction?
IF average construction with some air leaks and 2 or more levels, you only need a humidifier on the 1st floor.
Can you tell us more about the floor plan and where hydronic fan coils are located?
IF thsi is a sutom home, you might consider a central balanced ventilation system that has exhaust in the bathrooms and kitchen and uses a ERV or HRV (energy or heat recovery ventilator) and supplies fresh air to the return of each fan coil. Then you can install a humidifer (and possibly dehumidifer) on the main truck of that ventilation sytem to distribute the humidified air to each fan coil.
Otherwise if you have a 4 pipe system with seperate coils for heating and cooling on the fan coils, you could use steam instead of hot water for your heating and then have direct stem injection for humidification at each unit controlled by a solinoid valve.