Hmm... I have had both fed' and emp' rank at 100% for a few weeks. During that time I have done dozens of emp' missions and probably a few fed' missions too. I'm not in the bubble now, but I'll try this soon.
Well, speaking as somebody with some recent *ahem* "intense" experience of doing naval missions, I found that the rank-up missions would always appear wherever I'd been doing the missions to increase my rank.
They wouldn't appear as soon as I hit 100% but one would always appear by the time I'd done another dozen missions or so, which just goes toward your next promotion.
I've been in the position of spending weeks waiting for a rank-up mission too.
As far as I can see, the only real "trick" is to just stick to grinding rank in a handful of places and the rank-up missions WILL appear in those places eventually.
Suppose it's worth mentioning that somebody said rank-up missions appear more frequently in systems in conflict or in outbreak (presumably because the navy is present in those systems) but I'm not sure that's true.
For me, they just seem to pop up within about half an hour of play, wherever I happen to be working at naval rank.