I am just about hanging in there… 
I followed this tutorial: Unreal Engine 4 - Render Text to Texture using Canvas Render 2d - YouTube and managed to create a cube with some text on it. Then I set the material to Masked, removed Draw Texture and so only use the Draw Text function to write to the texture, which I use for both diffuse and opacity map. So now I have some text floating…
This leads me to the next question… how do I set the size of the canvas text rendering? Draw Text seems to render to the texture at a fixed size in pixels… I can change the size of the dynamic texture and this scales it up but it looks very low res.
Regarding the mouse action, I did get that working with an earlier experiment, using some onClicked event in the switch actor blueprint. I simply applied a small translation to the button (it was a push button) to show it in the depressed state, and toggled the visibility of a light. I guess for more advanced controls (like a rotary knob) i could have an invisible box on the left and right of the control to handle increment-decrement, would that be the right way to go?