I bought David's book on making trousers out of curiosity about expanding waists.
Despite David's assurance of the bottom corner of the pocket not kinking I have a feeling that moving 3 inches would. So, last night I tried to solve the problem in my dreams and worked out that using the corsetry 'bones' in the mouth of the pocket would take care of the kink because it would always stay 'stiff'.
Another interesting suggestion was the use of petersham stays under front pleats of the trouser so they don't splay. In cotton lycra the same idea could be extended by using a one to two inch wide tube of silk organza instead of petersham and stitching the ends to the seam allowances of the centre front and side seams.
I haven't yet finished reading the book, but it appears to be as valuable as his shirtmaking book

For David Coffin: What are bog posts?