How can a get a GUI drawn to a RenderTexture to respond to clicks .etc.?

I have developed a system for a CAVE-like immersive environment, which works by the following method;

• There are 6 cameras that capture forwards, backwards, left, right, upwards and downwards views of the scene. Each of these cameras renders its output to a RenderTexture.

• There are a number of canvases (one for each connected display) that each contain an image that displays a RenderTexture. Each canvas has one of the 6 camera views assigned to it, as well as a bunch of parameters for image adjustment (to control warping and edge-blending .etc.).

• The update method of each canvas takes the RenderTexture for the camera view that the canvas is assigned to, and blits it to the RenderTexture applied to the image within the canvas. The blit is done via a material that takes the image adjustment parameters to perform the warping and edge-blending.

Everything works brilliantly so far, but now I need to a a GUI with buttons and I have no idea how this is going to work. I'm guessing that it won't, being as the canvas isn't being rendered directly to a display or through a camera connected to a display. Please can somebody advise on how I could get this to work?