steerable dirigible, like the goodyear blimp, with suspended cable and basket?
A dirigible requires a temperature difference between the ambient (surrounding) air and the air inside the lift bladder. Or the same temperature but different density.
The heat of the lava would create an area of lower air density which would result in a significant loss of lift as it crosses.
I guess you could compensate with a high altitude crossing, but it still sounds dangerous.
If only Zuko had his fire fists or that of his sister Azula at that point it would have been easier.
Man Uncle Iroh is OP with the fire breath technique he teaches Zuko for the "chillers"
The problem will be the heat for a different reason: changing the dynamics between the gas in the device and the atmosphere inside making buoyancy much more difficult to maintain. Also, the engines used to move them around might be susceptible to the same concerns as helicopter engines & blades.