Is there any chance of getting hardware DXT texture support?
Something like glCompressedTexImage2DARB() in OpenGL where you pass width, height, other texture data and DTX compressed ByteArray: http://download.gna.org/gle/documentation/gle/0-3/html/group__GL__ARB__texture__compression.html

Unfortunately we do not control what GPU features that Flash Player supports. There is no support for native compressed textures right now, but there is support for Adobe’s own compressed format called ATF, which essentially deals with native formats behind the scenes.

So either convert your textures to ATF, or talk to Adobe about support for other compressed formats.