The Crye pouch may be the school solution here, on an unmodified night cap. I modified mine somewhat, which included adding two web and Velcro straps to the back, which creates a nice "molle" base for a pouch (not to mention really snugs the cap up on your head).
What I think would be the ideal solution here is to add in a pouch, which would ride lower, about mid-way on the back edge of the cap. More in line with where a C/W pouch would be a on a hard shell. This would give the pouch max leverage for counter weight purposes. If it's too far up on your head, you're not getting the full effect of the C/W.
Of course this would depend on the shape of your head; speaking for myself, getting it back and down, away from the crown of your head, just makes more sense to me.
As far as mounting, any kind of combination of webbing and Velcro might do the trick; the Mohawk actually makes a lot of sense, since you have the, uh, shaft to Velcro up on the top, which would allow you to run the pouch "low-ride", past the bottom of the cap. But if that's too much, then any good admin pouch would work, depending on how it attaches. Or can be easily modified to attach.