I'm working on installing central air in my house now and was thinking about using an cord and plug in for the service disconnect at the furnace end so I could unplug it and plug in into a generator. But my electrician neighbor says that won't meet code so I have to put in a switch. he says a normal light switch is fine, which seems silly to me as that would be much easier to bump and turn on while working on the furnace than an unplugged cord. So much for code helping.
I think hooking up a power source to the furnace would be much safer than trying to light it manually. There are two valves at least in mine, the 24v one that turns on gas to the pilot then the pilot ignites and heats up the thermocouple which turns on the main burner. So if something should happen to put out the flame the thermocouple would shut the main off and the control is set up so any time the small valve is open it constantly fires the ignitor so eliminate any chance gas build up.
The bad thing with inverters is ac motors don't run real well from them and they are quite inefficient (under 80%)