I am from the South, but I didn't think that it was a regional expression, until I just used to explain that I wasn't able to meet someone, and my correspondent - who is in British Columbia - thought that I had financial difficulties.
The expression always made sense to me, from the first time that I heard it, because in the rural areas, it is sometimes difficult to get from Point A to Point B. When you are in an isolated area, far from a major - or even medium sized city, it makes it difficult to get to other places. These isolated areas could be considered "pockets" because they are hard to get out of and so "out of pocket" refers to the difficulty to get from one location to another.
It is a helluva lot easier for say, someone from New York to hop on a plane to get to Paris than it would be for say someone from Long Key, in Florida to get to Orlando.
To me, "out of pocket" would not refer to being unavailable in an office, but more or less being in a different geographical area (pocket) that would be too inconvenient at the time to try to make it.
It's like if someone were in Mato Grosso do Sul in Brazil and there is a meeting in Adelaide, Australia, then that person would have to fly from Mato Grosso do Sul to Sao Paulo and from there to Miami and then to Atlanta and connect to Los Angeles and from there to Sydney and then to Adelaide by which time 36 hours have passed and the meeting would already be over - that would DEFINITELY be considered "out of pocket".