There are nearly endless ways of fitting a GPS to any bike using RAM parts - the only limit is your imagination! I drilled through my top yolk (1100S) and bolted a RAM ball to it as I considered this to be a far neater and cheaper solution than any of the expensive, ugly things you can buy elsewhere. If I ever sell the bike I'll just fit a replacement carbon yolk cover so the hole's not visible.
I realise that the question was about a 1200S but check out the RAM website at
http://www.ram-mount-uk.com/ and look at all the different fittings they do and you may well figure out a better way than anyone else has done

And not all solutions involve drilling holes - I've managed to fabricate GPS mountings for various bikes using RAM parts and none has cost more than a few pounds. The 1100S one is the only one involving any modification to the bike and that's just because this is my bike, not one that I've bought with the intention of selling on.
I'm not familiar with the 1200S yolk, otherwise I'd try to suggest a solution.