I can comfortably ride for 5+ hrs outside on a couple of different Selle Italia saddles. Indoors, I stand up for 10-20 Secs every 5 min. It gives me something to look forward to when doing intervals.

Outdoors, even if you aren't standing, there is a lot more movement going on as you maintain balance and go around corners. I wouldn't bother searching for a comfy indoor saddle. Just stand up as necessary. It's good for you.