I know when my own render texture is updated though (every frame), so there are two ways you can easily support this:
-In game mode, assume the render texture is updated every frame and re-render accordingly.
-Provide an API so the user can indicate when a render texture has been updated.
It is important that some form of support for render textures is provided because I use them to render vector graphics for electronic aircraft instruments. The xaml/svg is rendered into a render texture using NoesisGUI.