I recently had a really annoying creak/clicking sound coming from the saddle area.
Turned out to be the seatpost.
No matter how minor, after a few hours' ride even the slightest creak can drive you nuts! I hope you find yours soon!

Just one point - you say you lubed your seatpost. I'm assuming it's not carbon - or if it is, you used carbon paste and not standard lube?
Sorry, don't mean to insult your intelligence - just a knee-jerk reaction!