It's a lawyer problem, not a gunsmithing or money problem. Custom shop would be too much hassle and too little profit for NEF. They could, however, sell unchambered barrels to an authorized smith. Legal timidity is the only reason they don't.
Starting with a fitted unchambered barrel, the actual chambering would be very fast. Certainly one an hour and 4-5 with the right tooling and fixtures. Exclusive of insurance, because I don't have a clue about that cost, overhead should be less than half of gross, leaving the rest for labor and profit. $150k should be enough to set up such an operation.
The demand would be greater than one might think because there are a lot of people out there with flaming itches for cartridges that just are not available. These people may not have a high regard for the Handi, but were it the only choice available, they'd take it. Two obvious examples are the .25-20 and the .32 H&R Mag. The .30-40 Krag would make the cut, too.
The initial reamer selection could be rapidly expanded by a standing offer to chamber a barrel for the cost of the reamer.