I'm not sure if it would work (haven't tested it) but I think in theory if you just made the following macro
/cast Elemental Blast
/cast Unleash Elements
it would work. Macros that contain spells with a GCD in them only cast the first GCD spell then end when you press them, if you are in a spec that doesn't have EB, I would assume it would skip than and cast UE. Not 100% sure, but I'm 90% sure that would work.