RE: Best way to steam a bun
Ohman, you're sure not getting any help here, are you? There are many kinds of steamers and bun warmers on the commercial market, and you may even be able to get away with consumer market equipment for a small operation like yours. I haven't tried this, but I would give a big double boiler a try too. It's really simple, using a pot (with lid) on top of an open pot of the same size (with some water in the bottom one), over low heat.
Yes, I too prefer my dog and bun both grilled, but I understand that's not in the cards for you. Good luck.