Award Nexus (www.awardnexus.com), free up to a point for FlyerTalk members (click "more signup options" under the green Purchase button to get to the place to sign up using your FlyerTalk info), lets you search each of the three alliances through one or two airline sites, returning results in the same format in all cases, and does have a selection for Premium Economy in the cabin field.

But, of course, it depends on the underlying site that it uses as to how useful those results will be. PE is relatively new in many parts of the world (eg, the USA), and so not every airline in an alliance may have set up their PE to be searchable across the whole alliance yet. If so, that's a problem unsolvable by using another search site that in the end gets its information from the same place.