This topic comes up from time to time, and the usual consensus is "if it were that big a business opportunity, someone would be doing it".
If you think it's a money maker, go for it - but how much do you expect to make your mag bodies for? How much can you sell them for? How much would you be willing to spend for a brand new replacement mag body? Will you require the buyer to send in their beat up old pre-ban bodies so you can verify and destroy them before sending out replacements? If you can sell brand new high quality replacement shells for $5, I'll take 200. If you can't bring them in under $10, I'll wait for the ban to sunset, all my pre-ban mags to fall apart, or pre-ban prices to hit $30 before I'd bother.
With original pre-ban 30rd mags out there in the $15 range, you'd have to come in under that, and with the additional paperwork and records to verify receipt and destruction (in order to stand any realistic chance of avoiding the ATF hassling you into the ground), would it be worth it?
Any one of the existing mag manufacturers could start cranking out new, unmarked mag shells as replacements, and I think they would if there was any realistic expectation of a good profit and a good volume from it. If you think it can be done profitably, I encourage you to invest in it and give it a go, or even find some investors to spread the risk - but do the math on your business model first.