A waist stay is a length of ribbon or firm fabric that is inserted at the waist of a corselette or lining layer when constructing a fitted bodice. Its purpose is to help support the corselette, especially for strapless gowns.
I'm having trouble envisioning what you mean when talking about a waist stay for pants. It sounds as though you are talking about some sort of waist expander, perhaps at the side seam or back seam of pants.
I have seen pants that close in the pocket area where a bit of expansion does not make a difference in how straight or flat the side or back seams lie. I have also seen bits of grosgrain ribbon stitched inside pants closures with buttons placed about half an inch over to allow for a bit of expansion. Also, I've seen button loops made from rolled elastic. Do any of these ideas come close to what you want?
When making a waist stay do you need to do anything extra to the pattern you are drafting other than making a tube for the "stay"?
Where exactly are you thinking of putting the tube? Sorry for not seeing this more clearly. I think I need more coffee.
Hopefully someone else can come to the rescue.