There isn't a way to produce a keypress event based on the state of a locked key like that. There are a few reasons for that, but as a counter example, what happens if you copy something, then later want to copy something else without pasting? You select something in your editor, press Caps Lock, and rather than copying, which is your intent, it pastes the previously copied buffer, deleting your selected text. Hope you can undo!
I believe there's also no way in XKB to have a single keypress produce Ctrl-C, but there are a few tools to do just that.
Generally speaking, XKB works really well and is very versatile, but if you want something more custom than what it provides, it tends to make your life difficult. Or at least it used to, last time I tried.