In order to save cost on a prototype, I am going 2 layers board .
Most sections have been done on 2 layers and carefully selected ribbon wire strategies but now it comes down to the BGA, all my other component are QFP and every signal is well references and currrent loops on discontinuities are 1mm or less, 
Board is all digital, running at about 160 Mhz 2ns rise time, My traces are very short etc, a bit longer than 1/6th but not much. When I dont run a trace over ground I run a jumper wire on the bottom plane that has a grounded via on the jumpr ends, this wire  has ground underneath it  and top and bottom tracks (wires) cross themselves at 45 degrees or 90 degrees, depending on opportunities, as you can imagine the board required much effort etc to keep all short and tight etc.
However, once they come to the BGA region, there is simply not enough room to to use the bottom layer as a pure ground plane, and I dont feel like having jumper wires underneath the BGA for signal. There was gonna be a jumper strategy/flat cable, for the Power region in the middle of the BGA, I was gonna make it as less inductance as possible, however this is not elegant.
So I was thinking that maybe I can solve my board flex issue and my elegance issue by having made a very small 2 layer board that will accompany my main board and provide the necessary connections for the BGA power and they also provide a ground plane to any signal that may be routed from the bottom layer? Is a ground plane discontinuity of about 1-3 mm really that bad? underneath the BGA, or am I worrying too much?