So, how do I wire this into the hvac monitor? I assume I need to ground one of the output wires (24vac), also connect that ground to the hvac monitor ground? Does this make sense? If so, that answers my other question -- I have no choice but to connect the HVAC monitor ground to house ground.
I just received one of the HVAC monitors yesterday. They have a common ground for the four input circuits, and the ground connects to the one-wire ground. There is also a connector for the optional +5 volts DC for the one-wire system, and a connector for the one-wire data line. In addition, there are two RJ-45 connectors that you can use instead of the terminals for the one-wire connectors.
So, you are correct that you will have to connect one side of each of your 24 VAC transformers 1) together and 2) to the one wire ground in order to use this device. That should probably be safe as long as the 24 VAC circuits are not grounded on either side anywhere else. Because you tested with your voltmeter and could not see a voltage between EITHER wire and ground, the 24 VAC circuits do not appear to be grounded.
Note that, if you connect to ground at TWO places, you can create a ground-loop, which can generate a current and cause problems. Also, be aware that, as it is, if someone contacts one of the wires in your 24 VAC circuits or your one-wire circuits, no current will flow through them. However, with one side grounded, the potential would now exist for them to get a current from your 24 VAC circuits. Of course, if there is a fault in one of the transformers and the 24 VAC circuits become connected to the 120 volt lines, suddenly all of your one-wire circuits will become live, which you won't know until somebody touches them.
Another option might be an opto-isolator connected to the 24 VAC circuits, with the HVAC monitor connected to the output of the opto-isolator. However, the HVAC monitor is designed to convert AC to DC and cut the voltage to 9%, to reduce the voltage so the DS2450 can read it. You could, however, get the monitor kit and put it together yourself, changing the resistor configuration to read the voltage from the opto-isolator (and hobbyboards.com might be willing to sell you an HVAC monitor with that configuration).
I wouldn't try using the house ground as one of the wires in your "one-wire" system. Having the second wire in the same cable reduces the effect of electromagnetic fields that your cable travels through. Dallas Semiconductor has some very clear monographs about how one-wire works, and how to make it more reliable. I have been using CAT-5 cable, which is cheap, durable, and easy, and that has been working great.
It goes without saying that I TAKE NO RESPONSIBILITY FOR INJURY TO PERSONS OR THINGS RELATED TO YOUR FOLLOWING THIS ADVICE. I AM NOT AN ELECTRICIAN, AND THERE MAY BE NATIONAL OR LOCAL ELECTRICAL CODES THAT FORBID CONNECTION BETWEEN GROUNDS OR OTHER WIRES. ELECTRICAL CIRCUITS CAN BE DANGEROUS; GET HELP IF YOU ARE UNSURE.