Fortunately, this limitation has been recognized and mitigated by René Schule. René has initiated WriteableBitmapEx, which is a library of extension methods for the WriteableBitmap class. The entire project is open source (Microsoft Public License) and can be found on Codeplex.
I most urgently needed methods for rotation and flipping of the bitmap image, but there are also various methods for cropping, resizing, individual pixel editing, drawing shapes on the bitmap etc. All in all some very helpful extension methods for this specific need.