Vuforia QCAR : RenderTexture are black

Hi, I encounter problems using QCAR and RenderTexture under Unity 3.5.

I have my ARCamera. I get de VideoBackgroundTexture and set it as mainTexture of a plane which allow me to flip it on x-axis (my app will be used on a tv, so i have to mirror my video).

So, I have to change my MatrixProjection to mirror the ARObjects rotation too. Nevertheless, my VideoBackground is projected on a plane, and change the matrix doen't work in this case.

I have created another camera, whith the ARCamera as parent. I Create a RenderTexture for this Camera and make a GUI.DrawTexture of this one. On playing, my GUI print a black Rect... I have try to put my rendertexture on the material of another plane, but it seems to be the same issue.

Can anyone help me?

Best Regards, Yann