YES! Go for it!
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.