TL;DR: Line 2 is working, and after some initial hiccups they ported my number. Now I am paying $10 per month to make/receive calls and texts on my cellphone using my existing US number, while in Korea - as long as I have WiFi available.
Update:
So, I signed up for Line2 after querying them about porting my number. They said it was possible and to go ahead and sign up. Two weeks later, I signed up, paid the first month's fee, and requested the port. Tech support said my number was out of their service area and couldn't be ported???!!!?!?!?!
I forwarded them the e-mail where they said I could port my number, and they replied that it was actually possible, but they were having problems with carrier cooperation on certain numbers, so weren't sure how long it would take. I asked if they would do it any way, and about 3 days later, the number was ported successfully. Easy-peasy.
So far, I have been receiving and placing calls mostly without a hitch. I have had a couple delays or drops, but the WiFi router where I am staying also misbehaves with my computer, so I am not sure if Line2 or the router is to blame.
The App works great on my android phone for calls and texts - it basically just replaces my phone's dialer. The Web app makes it much easier to manage voicemail, greetings, etc than it was for any of my cellular plans.
As bad as my reception was at my house in the states, I may just keep this as my primary phone number and set it to auto forward to a cheap voice-only cell number when I am outside of WiFi range. We'll see how the next few months go. The total would be a few dollars more expensive than my old airvoce $10 per month plan, but actually receiving calls at home in my cellular dead zone would be worth it.