A bidding problem presented to me made me think.
You are dealt ♠109x ♥x ♦Axxx ♣KJxxx, and partner opens 1♠. This is a nice tweener hand, contingent value, that we often see. If you use a Mini-Splinter approach, great! But, often this is not the case. So, you undoubtedly start with a forcing 1NT, right?
Partner now bids 2♠.
This is getting interesting, now. A possible hand for partner could be ♠AKJxxx ♥xx ♦xx ♣AQx, right? Is that too much? Well, what about after a 3♠ rebid, instead?
In either event, you want partner to have a COV (concentration of values) in the black suits if you want this to be a fair slam gamble. A similar hand for Responder might be to remove the club Jack and replace it with the spade Queen.
As you can imagine, a simple splinter does not solve the problem. Partner cannot tell which minor needs a COV.
I have not thought through, yet, whether this is the most useful technique, or whether this is a net positive or a net negative (the value of the needed disclosure for the rare slam minus the cost of the disclosure when it does not matter). I have also not yet thought through when this idea might be of use, meaning in what auctions. However, it seems that the solution for Responder is to have a call that asks Opener if he has a COV.
This could be done in one of several ways. One technique might be to use a jump to 3NT (if not needed) to simply ask partner to bid a side COV if he has one (3♠ after heart agreement; 3NT for a spade COV). That would save jumps for some other meaning. A second technique would be a bit more straight-forward, and perhaps with less disclosure. That would be a "fit-jump" where the fit jump could be thought of as showing this type of hand or asking this type of question.
The key is to determine when, if at all, this makes sense. For example, one might decide that the auction 1♠-P-1NT!-P-2♠ can never result in a slam unless Responder has the hand type described above. If so, then 4♣, 4♦, and 4♥ should be fit-jumps, seeking a COV in spades and this suit from Opener, but only, of course, if the potential gains outweigh the potential costs. That's a judgment call, as always.
I'm still undecided about this idea, but it seems intriguing. It is somewhat related to the theory behind the Empathetic Splinter, when the ES is used in what appears to be an invitational-only sequence. Responder is showing the side source of tricks and is, in a sense, asking if Opener has an Empathetic Splinter for both side suits; a COV is also a COW (concentration of weakness).
Makes me think...