I guess I find myself not too interested in pulling this off, or polluting the code enough to accomplish it. Either way, you are going to be installing this software on other machines. Can either bundle .Net 4 with it, or do something like AOT it with Mono.
I'd be more interested in making it compatible with Mono.
After looking at the code last night that was my general feeling also. Getting it to compile under Mono would be more groundbreaking than backporting to NET 3.5.