A friend suggested an interesting treatment. Of course, I had to expand it even more.
The problem. Blah-blah-blah occurs. The opponents have shown spades. We have hearts. Partner's last bid was 4H. We want to explore slam, but we have not together worked out whether spades are controlled. What now?
My friend's idea makes sense. 4NT is RKCB. 4S is asking for a spade stopper and is RKCB if you have it. Thus, partner's replies to 4S will be:
4NT = 0/3 with a stopper
5C = 1/4 with a stopper
5D = 2 without the Q but with a stopper
5H = no stopper
5S = two with the Q and a stopper
I think this can be deepened even more, in a Meckwellian manner, if so inclined. You could also add in an immediate-answers with or without. For instance, Responder's 5C call could be 0/3 with a spade control. Opener could bid 4NT after 4S to ask or answer at 5C+. Something like that.
Or, a uni-directional. If the mover is weak, 4S asks, but 4NT+ answers with. If the mover is strong, 4S asks and is RKCB, 4NT asks and shows, 5C+ ask other questions.
In the end, though, I think the simple suggestion is easy and good. If 4S invites slam without a spade control, Opener answers RKCB with a spade control, skipping 5H as the decline bid. 4NT, instead, asks RKCB but shows a spade control. Simple, and effective.
BTW -- this solves a Bridge Bulletin problem (as noticed by my friend, Ken Eichenbaum).