After reading your post, I did the same, calling our closest Apple Store that is 45 miles away. I explained that I had reserved a Verizon 6S for September 25 and asked if I had to have a Verizon account or if I could activate the phone with my Selectel SIM card. The specialist said that they had been told that phones were locked (at least the early ones), that if I had preordered a Verizon phone, then I would have to have a Verizon account. She said that she couldn't say for sure that the Selectel card wouldn't work, but she was doubtful.
I really cannot fathom this. Do the major carriers have this kind of stranglehold on Apple that they cannot sell a reserved iPhone on launch day to customers that use MVNO's? I've also read where people went into Verizon stores on launch day and bought device-only Verizon iPhones.