I've been lurking the threads for about 3 years now, slowly maintaining my outback (2.5i shhh don't tell anyone), but never had anything to contribute.
I recently went to the junkyard to look at the selection since my engine is going out but got distracted by a leather interior 2006 that must have been really cared for. an hour later I'm out of there with the seats, carpet, and some other various bric-a-brack. Some of you know where this is going. Long story short the 2005 system does not like 2006+ seats. I looked through the forum and only found methods for faking out the side airbags with resistors, or trying to use both the 2005 and the new seats hardware, or shoving the new upholstery over the old seat.
It took a few days of thinking, but I came to the conclusion that all that is needed is to make a harness to interface the new seat's occupant sensor to the old connection, swap the SRS module in the dash for the correct year, and change out the seat belt for the 2006+ style.
I made this quick little guide while looking busy at work today: 2005 Seat Swap 2006 2007 2008 2009 airbag fix.pdf
Hopefully this makes the process easier for someone else in the future. If you have any questions let me know!