~face palm~ and there it is, thanks muchly.
Could also go with using the devastator doctrine on Ultramarines to give snapshot re-rolls (which marginally more than doubles your chance of hitting sans-skyfire). With a Devastator squad and a few combat squads with heavy weapons it should be possible to down an aircraft in that turn. (plus if you are going second and the flyer comes on at first opportunity, this is probably the perfect turn to use that doctrine anyway)
Additionally, it doesn't specifically say in the BRB that Flying MCs get to choose to use Skyfire, but in the FAQ, page 6 (of version 1.5 at least), it does say "yes, FMCs should get to choose Skyfire while Swooping". So there's where that rule was hiding, if anyone is curious.
Thank you for voxing the Church of Khorne, would you like to donate a skull to the Skull Throne today?