Note - this is using the app on my Sony DVD player - not sure how different others are,
For searching Prime, I add videos to my Watchlist typically for the sole purpose of serving as categories. With those videos highlighted (but not selected for 'more details'), I click 'Options' on the remote, which brings up a submenu with a bunch more options, they key one being "More like this". At that point, you'll get 50 similar videos, almost all of them Prime (the ones that aren't I think have probably recently dropped off, since the "Prime" filter is enabled).
Even now, when I add something new and unique to my Watchlist, I'm still amazed at the stuff that shows up on "More like this."
There's a good chance I might've cancelled Prime (or at least been upset about the stealth renew) if I hadn't discovered this.