In reverse order:
- There aren't any ace libs that deal with rotating that I'm aware of.
- The WoW API supports Affine Transformations for textures (and textures only) which includes the ability to rotate them.
- You might be pleasantly surprised in a few weeks :)
I would be SO happy if you would support texture transformations via SetTexCoord. Right now, for example, I have 6 panels (with 6 different textures) that are all mirrors of each other. I have TopRight, TopLeft, TopCenter, BottomRight, BottomLeft, BottomCenter. If you implemented transformation, I could use the same single texture (rotated 90deg, flipped, etc.) for each of the corners and a second texture for the two centers.
For rendered content, Blizzard often reuses the same textures in a number of creative ways to create different-appearing results. I can imagine a lot of users doing the same things for 2D content in their eePanels. Just make sure you put these settings in advanced controls or lots of people will mess up otherwise good-looking panels by experimentation and not have a clue how to get them back.
Oh and get ready for lots of fun matrix math for transformations! :) It might be easiest to start with two simple transformations such as Flip Horizontal and Flip Vertical. Then work your way up to Rotate 180deg, Rotate 90deg CW, Rotate 90deg CCW. If you're still feeling adventurous, you could add Rotate Arbitrary, Shear Horizontal, Shear Vertical.
P.S. -- There's an awesome tutorial on WoWWiki with a pre-built transformation function you should look at: http://www.wowwiki.com/SetTexCoord_Transformations