Yes. There's absolutely a demand for a steel (or even aluminum) framed camper that is insulated and built to order.
I have a Callen on my '07 long bed (8'0 bed) Tundra- talk about a unicorn shell - I lucked out and found my shell on Craigslist for sale up in Idyllwild for.... are you ready for this... $500. It was in great shape owned by an older gentleman who could no longer crawl into or out of it.
We love the shell and take it on 3 week trips to southern Baja every winter- we've wheeled it out to the end of Punta Rosallilita- it's light enough that we can still drive across beach sand (in 4x4) although I did install airbags to help level it out on the truck. I've fit out the camper with quick connect propane for an outside grill, quick connect outside water hooked to a 30 gallon water tank with an on-demand propane hot water heater. There's also a Thule awning I mounted onto the camper with a big aluminum bracket I bought at that Industrial metal shop up off Convoy. We also found a fully galvanized rack with expanded metal that fit (with some modification) our camper for $200, we drink coffee up there and call it the Aloha Deck.
If you're seriously going to pull the trigger on a Callen revival or go it on your own- I'd be interested in hiring you to freshen up my Callen- and even fabricate a couple of things for it that I don't have the skill to do. I'm also dreaming of taking the bed off my truck and installing an aluminum service body onto it AND then mounting the Callen onto that- or if you're up for it, having a Callen-style camper made to fit on a flat bed tray.
If that's the kind of business you'd be offering- I for one would be knocking on your door. We love the "light and fast" approach this kind of camper offers and we like the comfort of a hard sided, insulated camper- especially in central Baja where it can blow like crazy for weeks.
I'm also in San Diego - if you'd like to chat sometime about Callen campers or this stuff in general PM me - I love talking camper stuff.
I think there is a market for the Baja style camper. I like what Northstar is doing with the Vista and I hope it is a success for them. What I'am looking for is a long bed version of the Vista for my 1 ton reg cab dually Ram. It must not be longer than the bed of the truck since I plan to tow a bumper pull travel trailer and I want to keep the overall length of rig as short as possible.
To the OP I would be interested if you decide to go ahead with building campers, although it will be a year or so before I will be ready to buy a camper need to pay off the truck first!
I will try to get some pics over next weekend...it is currently being used as partially intended...to haul crap to the dump. I have a bunch of yard waste loaded at the moment ready to take down to the dump next weekend. Sweep it out and ready to throw the camping gear back in. The foot print is the same as my Dodge RAM truck bed. I am 5'-10" and can stand it with just a slight bend in my neck.