Thanks for all the advice. I talked to our corporate travel office about it this morning, and I was told I could fly to Seoul and take the train if I wanted. I really liked the flights I had to Seoul, so I decided to go ahead and do it. One big bonus is that when the travel agent went to book the flights, the premium select / domestic first fares came out cheaper than coach. I had been planning to buy up on the long flights, so I can't complain about getting it for free. I'll be arriving really early (4 AM) on a Tuesday morning, and I don't have anything to do in Busan until dinner. Hopefully the train will provide a bit of adventure to help pass the time. On the way out, I'll catch a flight at around 6:30 PM in Seoul so I shouldn't have to get up super early.

I won't be going until June (so no cherry blossoms), but hopefully there will still be some scenery to watch. First class on the KTX might be a bit tricky since they'll only reimburse me for the cheapest fare. On a plane ticket, it's easy because there's already a receipt for coach and I just pay the upgrade price out of pocket. I'll have to ask them how to do it if I decide to go with the first class.

BTW, are you saying it's possible to connect through Seoul, but not do immigration until Busan? I had assumed it would be like my trip to Jeju last year: clear customs/immigration, drag bags on the bus to the domestic terminal, re-check bags, go through security again, etc. Doesn't matter for this trip, but if I ever travel to Busan again it would be good to know.