I didn't see anything on creating plugins. The closest I saw was
Because you can load and run some GUI programs inside or on top of WinPE, you could conceivably add your favorite apps to the WinPE environment for an ultra-small runtime for use on a netbook, a PDA, or another limited-resource environment. Compact browsers such as Opera and QtWeb or file managers such as Salamander have been reported as working with WinPE, as has Windows Explorer itself (I find lots of evidence that people have made it work by importing it into program files on the image source, but I haven’t been able to make it work).
Just copying them into program files usually isn't enough; registry entries or drivers may be needed. Of course, I may have just missed those details in this guide.