Welcome to the forums, Ben! You will find there are plenty of open-minded, creative thinkers here that can help guide you in your endeavors. I encourage you to start pouring through the forums and reading past threads relevant to your situation. There are literally mountains of information at your disposal.
The dehumidifier-turned-water-heater would be a good engine to retrofit an existing refrigerator into your boiler loop. The external heat exchanger could be rerouted to a refrigerant-to-water condenser hx, which could dump the waste heat into your boiler loop. It would only add a thousand or two BTU per day, but it could work. Whether or not it is worth the effort is all up to you, but it would give you some experience and confidence to move into the full-sized realm.
Tell us more about the multi-zone, multi-temperature boiler loop setup you have running. Natural gas use is incredibly easy to track and log, so if you have been operating this rig for a few years or longer, you should have some good historical data. Even if you have only one gas meter, and haven't been logging each zone's usage. Your gas bills will tell you volumes with respect to system retrofit or supplement options. Due to the fracking craze, natural gas is incredibly inexpensive as an energy source today. Like all good things, they will end eventually, so it is a great idea to have another energy source to fall back on when this eventually happens.
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