I agree that the only place to add weight without changing the design is to use inserts like the Tubbs Carrier Weight System. However, shortening the hammer would allow the top of the carrier to be enclosed like a tube.
Jim said he increased the weight of the bolt carrier by adding an anti-bounce weight (to reduce bolt bounce). He then stated that he "almost doubled the weight of the cycling components" (6:37). He didn't double the weight of the carrier....but the cycling components.

