Since rad onc is your first choice, I would pick maybe 5-6 programs and not apply to other programs at that hospital. Pick 1-2 really strong programs and 4-5 mid tier programs and apply only to rad onc. Then apply to the rest and go for it.
If anyone brings it up, what I would do is just say honestly that you love rad onc, but you recognize how difficult it is to match, and that it has gotten extremely competitive. While perhaps radiology isn't your first love, you would prefer (maybe loans are an issue?) not wanting the uncertainty of not having a job. there are elements in radiology that you love, such as technical aspects, technology, physics, thinking in permutations, imaging, that are present in both fields.
The trouble is if no one brings it up but there is talking behind the scenes. Well, there is nothing you can do about that... however, be comforted that it will only be a percentage of those hospitals. I have a friend who applied to both rads and medicine at ALL hospitals, and he got his first choice. No one brought it up.
Heck, you could probably just say you're coming for the transitional interview if anyone from rad onc stops you and asks.
I personally have a different view of the world and medicine, and I happen to think they are more related than people would think. It's true rad onc is patient centered and radiology is not (although there is interventional and some procedures like thoracentesis in diagnostic), but I guess I just have a different way of thinking about things.