Yeah, the brass issue really isn't an issue, but his barrel question is. 38-72, have you found anyone who makes a 0.321 barrel or would a 8mm bore diameter barrel be okay for you, especially if you want to use cast bullets and could have more flexibility on bullet diameter? Also, the 30" length requirement may be an issue as that would further limit the likelihood of finding a 32-40 takeoff barrel that could be used or a 32 winchester special barrel that could be set back and rechambered.
I seem to recall MidwayUSA or Brownells listing a Green River or Wind River barrel in 32 caliber, but maybe it was for a 32-20. Have you also asked any of the rebore folks if they do that bore size? Finally, I guess there is the option of getting a liner installed in any suitable barrel you can get, but I don't know if the 30" length will pose a problem there. Sorry no answers, but I am curious to what you find out. - Mike