If you're talking about double-diapering, you may want to look into the boosters we use with the cloth diapers. They're essentially absorbent strips of cotton, and I can't think of a reason for why they wouldn't work with disposable diapers -- except for having to extract and launder them, with all the related infrastructure that implies (as in, a covered container of some sort to put them in until you do wash them).

Mainly suggesting it because there doesn't seem to be a good disposable solution.