the action button was a kinda of a love hate deal, i thinks thats why no one has tried to implemented back in, coming from manila 2.0 that is..
to me a tab will not work but if you slide out the ke board then you should get the action screen in landscape, that is the way I think it should be implemented for now till HTC release there new version of TF3D
here it is:
http://forum.xda-developers.com/showthread.php?t=515421