I know LibrarIan has a Verizon Galaxy S3 on a GSM MVNO, and IIRC, the GSM end should already be carrier unlocked for the US due to the whole FCC/Verizon LTE spectrum purchase agreement. It should just work... but it might depend on when you're putting the SIM card in (either before or after turning on). There should be no fancy flashing or unlocking necessary.
Thanks for the info, I.P. So do you think that "Phone activation: This SIM card is from an unknown source" notification is indicative of an actual problem, or just a nuisance message?
The only option I can think of off-hand is to either dig through the system menu for network preferences or dial *#*#4636#*#* as that too should bring up a menu option to toggle between preferred network types. If you're set to CDMA instead of GSM and it's expecting a Verizon LTE SIM card, that would likely give you the error message you're seeing.
The network preferences is pretty easy to get to (Settings -> More Settings -> Mobile networks -> Network mode), and that allows me to choose between Global, LTE/CDMA, and GSM/UMTS. By default it was set to "Global". I also tried switching to GSM/UMTS, and that warning notification remains.
Also, FWIW, I inserted the ptel SIM while the phone was powered down. I've tried power cycling several times, even after switching network mode. The notification remains.
I also tried the dial code you suggested... is there some trick to doing that? Or do you just punch it in and dial like a regular number? When I did that, it actually tried to call out.