First, since this is such a high altitude pov, how does one fade blue sky to deep blue or black as altitude increases?  In bryce or post?  How do you do it either way? 

second is for opinions only.  option 1)  high altitude plane, perhaps SR 71   2) high flying, high tech globe circling type ballon...  this is the way I'm leaning..     or  3)  on the sinister side, a bomber flying away fm mushroom cloud. ....   my last choice personally

sky is fm sky lab manipulations plus two additional sky planes, two volume spheres, and camera inside a glass sphere to give curvature. 

I really need help on fade to black thing.

Honestly, I'd throw in a large moon and a slowly transitioning starfield. But thats just me.

But of the 3 choses you suggested, I'd do the balloon. But keep the detail of it low as not to overpower the planet/skyscape.

 Personally, I would use gradient layers in Photoshop, much more control over color & opacity. Plus one less component in Bryce willl reduce render time.

Have fun!!
ICM