My advice would be to buy some new brass when you get your new barrel installed. That may sound like a smart ass comment, but it's really not - I have the same dilemma as sooner or later I intend to get a aftermarket barrel for my G29 and noticed the same thing you did - a bulge on the shoulder closer to the rim than the reloading dies can get to. I expect I will have to buy a new shipment of brass to start reloading for my aftermarket barrel - all things considered, it's not that much (last time it cost me $80 for 500 new brass [from topbrass]). I have no ideas on how you would be able to get that last little bit straightened out - but if you do find a way please post it on this thread!
"What's the down side to 10mm again? Oh, that's right, there isn't one."" Carrier21