It's pretty simple if the DAC needs a 12V input. I have a Beresford TC-7520 and, like Tobias, I've run it directly from an SLA batttery.
With some wire, a couple of spade connectors for the battery terminals and a power plug from Radio Shack to match the one on the AC power supply for your DAC you're all set, as long as you don't mind manually recharging the battery when needed.
With some wire, a couple of spade connectors for the battery terminals and a power plug from Radio Shack to match the one on the AC power supply for your DAC you're all set, as long as you don't mind manually recharging the battery when needed.