First of all, you don't need to parent to UIParent, it should be already parented here by default.
This is the reason why FramePositionManager taint if you move it from it's default position. Enabling ignoreFramePositionManager should solve the issue with tainting if parented to UIParent and if you move it from here.
There is another way to avoid tainting. Tuller seem to use this method aswell as seen on page 1 of this topic. On PTR, what I did when saw this button on a mmo-champion stream was to parent ExtraActionBarFrame to an "holder" frame. You use your own frame for StartMoving() and StopMovingOrSizing() instead of the Blizzard Frame. When you parent a frame somewhere else than it's default parent, it ignore FramePositionManager. This method is almost the same some users did in here with their regular action bars (MultiBarLeft, etc).
Btw, thx Roth for the texture hooking idea, I agree that it was very ugly.
