Thanks, rjbull. What I really want is one more hotkey, for copying. So if I press the normal set of hotkeys, Ctrl+C and Ctrl+V, it will make a normal Copy&Paste&Forget, but if I copy by pressing the other hotkey, it will Copy&RememberForEver. Maybe they all can do this already, but as I implied, I don't tend to figure out what is not obviously.
This may not be possible to do reliably. When you press Ctrl+C, it is not the clipboard extender that does the copying - it's always the app you are working with. Windows then notifies the extender that something new has been put on the clipboard, so the extender can grab it. But at that point the extender cannot know what keys you pressed.
In order to achieve what you want, the extender would intercept your "special" keypress, but how would it tell the app you're working with to do a copy now? There are several ways, from posting a WM_COPY message to faking a Ctrl+C keypress, but they aren't 100% reliable. It would work some of the time, maybe most of the time, but sometimes it would fail and cause frustration.
Let me reverse the question: why would you want to bother yourself with having to decide whether you'll need a clip later or not? Why would you want to stop and think about it (even if for a split second) every time you are about to press Ctrl+C? Let the computer worry about that, I say!
The solution for me was to pick a clipboard extender where there is no penalty for storing lots of useless clips. One brain-dead program I absolutely must use for my work (since it's an "industry standard", no less) uses the clipboard for its internal operation. As a result, every sentence I translate ends up on the clipboard and in Ditto's database. Useless, yes, but it's not a problem at all, since Ditto is really fast, the db is not kept in RAM, and it has an instant incremental search which narrows down the selection to the letters I type. So I can bring back the clip I need by typing faster than I would through scrolling and eyeing the list. I just stopped worrying about the crud in the database, since it doesn't cause any problems really.