As the song goes: Let's start at the beginning, it's a very good place to start.
The service interval on a 1200 series bike is every 9,600 km's (6,000 miles).
Your planned trip is 12000 km's or 7,500 miles.
You are perhaps planning on overstepping the service interval by 2,400 km's (1,500 miles).
Will the bike die if you miss the service due date and overun by 2,400 km's? No
Will the bike need a top-up with fresh oil during it's 12000 km journey? Probably yes
May there be other things to check at what would be the normal service interval date? Yes, the brake pads for instance. But I guess you will be checking these sometime on your journey anyway.
Is it easy enough to change the oil and a filter when away? Yes. See the sticky on tools that are wise to take on a reasonably long journey.
http://www.ukgser.com/forums/showthread.php?t=134046
Will you need an oil filter tool? Yes. You can always drive a screwdriver through the old filter to use as a lever to get it off. This method is less satisfactory for a new filter going on.
http://www.nippynormans.com/product...ench-r1200gsrtstsrk1200rsf800-series-oil-1200
Will you need something to catch the old oil in? This depends on your environmental greenness and social conscience. As they say in Life of Brian, "Don't wear your best sandals".
Will you need to take a full container of sufficient fresh oil with you? Maybe yes, if on the day you chose to make you oil change you cannot buy any.
Can you pre-book services away from you home country? Yes. Do people do it? Yes. Can you just turn up at the door? Yes but they may not be able to help you just at that moment.
Where / how do I find a dealer in a foreign country? Via the BuMW Motorrad webpage and / or a Google search.
http://www.bmw-motorrad.com/com/index.html
Have a great trip.
PS Where are you going and for how long away? I am very jealous.