Sure, you can use subpixel rendering for arbitrary images. However, subpixel rendering is really a generic 2D image processing technique — it has nothing to do with ray tracing specifically. You could just as well use it with any other 3D rendering method, or even with a simple 2D drawing, photograph or even video.
Thus, I'd say that "subpixel rendering for ray tracing" is really conflating two distinct problem domains that are best treated separately. About the only relevant connection is that, if you're ray tracing the scene in real time and know that the resulting image is going to be drawn on the screen using subpixel rendering, you can use this information to optimize the pixel density (and aspect ratio) of the intermediate image (e.g. using 3x horizontal pixel density for a typical RGB LCD screen).
A potential source of confusion may be that, on current computer system, subpixel rendering is commonly used only for text, and is typically integrated into the font rendering code. The main reasons for this are arguably historical, but it's also where the biggest payoffs (in terms of visual improvement and readability) typically are.
Also, due to the way text tends to consist of simple and repetitive vector shapes, integrating the subpixel rendering into the font renderer offers some extra optimization opportunities over just rendering the text into a high-resolution buffer and then postprocessing it.
That said, I fully expect that eventually, as technology matures, we'll move to a system where subpixel rendering is simply done transparently by the GPU, or possibly by the screen itself.
(This will most likely require applications that want to make full use of this feature to deal with physical pixels that are smaller, and not necessarily the same shape as, the "logical pixels". But then again, we're already moving in that direction with high-DPI screens.)