If you are regulary visiting or selling games on FGL you may know that they have started to offer their own Ads service. In fact it is now mandatory to use their ad system to benefit from their distribution network. From what I have heard they should be quite good though with good Cpm.
Have anyone with the skillset looked at the api? http://flashgamedistribution.com/fglads/FGLAds.html
I have 0 knowledge on how to build an extension, but from just looking it looks kind of basic? How complex would it be to make an extension for this?