Electronic – Simple AC line state detection for digital controller input

Last year I moved into a house that has an old ('50s-'70s vintage) GE low voltage lighting control system. It uses 24VAC momentary wall switches to toggle solenoid/latch relays that feed 12 main lighting power runs. I want to do some computerized home automation using this system and most likely something like a Phidgets digital I/O controller.

Controlling the lights should be easy. I just need to piggyback momentary circuit closures in parallel with the existing switches. This should be easily achieved with a digital output controller. Although, the Phidget board I'm looking at says it can control "up to 30VDC" so this might be a problem since I need to switch an existing 24VAC signal.

I also want to be able to detect the state of the main lines to know (in software) what circuits are live. The simple answer is to put a relay in parallel with the main load. I can wire in the digital I/O controller to the switched poles which then tells the software the state of things. The problem I'm having is finding appropriate (e.g. low load/cost) relays that work with a 120VAC coil voltage. There are plenty of heavy duty industrial relays, but those are far beyond the needs (and, more importantly, cost) of my application. The switched side needs to handle only a very low DC voltage supplied by the I/O controller.

I'd settle for a mechanical relay if that's what it comes down to. (Also still looking for a suitable candidate in that area.) But I'd like to avoid having a bank of 36 mechanical relays just to translate the 120VAC input signal and 24VAC output signals to/from the controller's DC input/output. However, my research so far is suggesting that might be the only way.

I've been searching various suppliers catalogs for solid state relays that will do this, but no luck so far.

I've also thought about using analog I/O controllers with voltage sensors, but that seems excessive/unnecessary and I haven't found a cost effective solution there either.

Can anyone recommend a relay product for this solution?

Or does anyone have another suggestion to this problem?

Update: Change link to a GE document that somewhat represents the system I have. Document includes schematics of the relays and other parts.