To attach equipment, you can connect a 24VAC-friendly load between (e.g.) Y<->C or W<->C.
Problem with attaching a load is that if you want to build a standalone unit with the nest, you’ll need some type of inverter when powered off DC, or perhaps a simple sine wave generator? I’m more of a digital, DC power guy.
There might be a way to trick the Nest into running on something closer to DC, but I could only guess.
To attach equipment, you can connect a 24VAC-friendly load between (e.g.) Y<->C or W<->C.