Yeah, normally I wouldn't even bother to try guessing what a song would be about without actually hearing it. Especially with just the title and the songwriter to go by. However, as you've already pointed out, it's a song solely written by Bruce and I do think the title seems to have that sort of edge to it. Could be anything though, so we'll have to wait and see.
I just have a feeling that the song being about air-travel might be a bit too easy a guess, even though this is Bruce that we're talking about here.
Bruce has a tendency to write a lot about religion and the potential fallacy of the afterlife. He also certainly has a love for writing about the skies, space, the world above but not always the world beyond.
My wild speculation is that If Eternity Should Fail will be more straightforward Bruce writing: calling religion and heaven into question while searching for hope in our mortality. Empire of the Clouds probably started as a song about aviation, turned into a philosophical pondering about the nature of human buoyancy and how that relates to the concept of an eternal soul, then became a theoretical brainstorming session on how man would live in the clouds, then turned into a question of man and God being one and the same, which snowballed into a final verse about dogs licking rainwater off a windshield.