Rockbox has no volume limit I know off. The best you can get is the EQ precut, and as you said, it doesn't apply to Radio or anything else that doesn't go through the equalizer (like any games/plugins that may have sound, but you can delete those plugins).
You would need to modify the Rockbox source code and compile it yourself in order to place a volume limit.
In general, and from my point of view, the Rockbox community has shown to not be in favor of a volume limiter that doesn't let the global volume go higher than a certain value, however, I don't think anyone has thought about special situations like this one.