If I have a device powered by 8 AA batteries, can I safely use a 12v wall wart to power it? If so, would I have to wire anything extra into the circuit or is the output from a 12v wall wart identical to 8 AA?

It really depends on the adapter that you want to use. In most cases, a 12 volt adapter is unregulated and can be anywhere from 9 to 15 volts. If it is an actual 12V adapter, say about 1000 to 1500 mA, it should work just fine. AA cells can provide up to 5 amps at times but they would not last long. I some how don't think that they were called upon for that kind of current draw as battery operated devices are designed to run for long times. Even a 150 mA adapter might work if the needed current is low enough.