This plugin provides an in-game mechanism to define regions in a worldedit-like fashion.
Simply type /pwand to obtain a worldedit-like wand, select two points using left- and right-click, and type /pregion to output the correct bounds of the region.
The problem with worldedit is that the bounds it gives you use a different system from PGM's region-defining system, giving you inaccurate bounds. Thus, this plugin is a simple and effective solution to expedite the process of defining regions for your PGM maps.