I think this idea could be more hassle than it`s worth Unless you can fit the 12`s pannier racks onto the 1150`s frame and line them up so the exhaust goes through the cutout ok I can`t see how they can possibly fit on.
I don't think the rails would be the real problem (mainly because the 1200 Vario don't use any), it would be the 'mushroom' buttons on to which the 1200 Vario's lock that would be the difficult part.
I hate top boxes too.......but I use one every day of the week for commuting. There's no other way to carry both my stuff and my wife's stuff (we commute together) without affecting filtering ability.
I take it off at every (weekend) opportunity, but as much as I hate it it is very useful.