After discussing general principles with a regular partner of mine, I realized something rather stupid.
My general rules had been that a leap to the five-level was Exclusion RKCB if I could be short in the suit but, if not, then RKCB for this suit. In other words, assume a cuebidding sequence where spades are trumps. 4NT would be RKCB for spades. Obvious. However, what is 5♣? In some situations, such as where I could be short in clubs, 5♣ would be defaulted as Exclusion RKCB. If, however, I am known to have length in that suit, then 5♣ would be RKCB, except that partner is expected to treat the King and Queen of clubs as the key cards and ignore his King and Queen of spades. This would typically make sense if, for example, we already know about the spade secondaries because of trump cues, or because I as the asker am looking at them.
My partner suggested another level to this general rule, one that made me say, "Duh!!!"
If partner has already denied, say, a club control, then what the heck would be the purpose of 5♣ as Exclusion RKCB? Similarly, if we have already shown all of the top three cards in clubs, or have denied these, then how can 5♣ be asking that question?
The rules should make more sense.
I still believe that the general defaults should apply. If, however, the normal default would make no sense, such as if the default would suggest Exclusion in a way where Exclusion would be stupid, and where RKCB in that suit would be illogical, then the bid should mean something different.
What precisely these should mean are worth considering.
It dawned upon me that one reasonable default is to look to the next suit up, using the "useful space" principle. Consider as an example a sequence where spades are agreed as trumps. Suppose that the normal defaults would suggest that 4NT is RKCB, 5♣ is RKCB (but answer club secondaries), 5♦ is Exclusion, and 5♥ is RKCB (but answer heart secondaries). If 5♣, however, would make no sense as RKCB for club secondaries (because Opener has denied the A/K/Q of clubs) or as Exclusion (for the same reason), then look one up. Bump the 5♦ meaning down on, to save space, such that 5♣ would be Exclusion now, but excluding diamonds. If that meaning also make no sense, bump down the meaning of 5♥ to make 5♣ be RKCB for the heart secondaries. If the 5♦ meaning is bumped down to 5♣, then the 5♦ call now would likewise grab hold and pull down the prior 5♥ meaning.
This might be a tad difficult, as then partner, if hearing a 5♥ call, would have to think through whether 5♦ or 5♣ would have made sense, and similarly you would have to assess the same thing before bidding the otherwise obvious 5♥. A safer technique might be to have an impossible ask (like 5♣) always flag the highest other option. How would this work?
Well, consider the example again. If 5♣ makes no sense under the existing defaults, then it shows the normal 5♥ meaning or, if that makes no sense either, the 5♦ meaning. This is safer for a few reasons. First, if you forget or do not realize that 5♣ would have worked, you bid your normal 5♥, risking only partner looking at you scornfully for forgetting that 5♣ would have worked just fine. If you bid 5♣, partner knows that this makes no sense and then knows to figure out what you really mean, according to the agreements. Plus, perhaps most importantly, the 5♦ call is unaffected, such that you do not have to think about the possible meanings for 5♣ as asker or answerer to a 5♦ call.
The second way is much safer. The first way technically seems to have more options, such as defining cleared bids at the top in new and creative ways.