How about this: Cold-set the frame to like 140mm. It's actually really easy and you can do it yourself at home with some careful measuring and a big two-by-four for leverage. Cold-setting is actually just a fancy term for somewhat carefully bending your rear stays to a slightly different position. (This is assuming you have a steel frame. Carbon, aluminum, etc. are a whole different ball game with regards to cold-setting)

With the super-wide frame, now score yourself a simple flip-flop hub with 120mm spacing and an extra-long axle (Formula, etc.) and space the non-drive side out to get an off-center 140mm spaced hub. The 42mm chainline now becomes a 52mm chainline, and you have one weird-looking rear wheel. The wheel will have to be dished to get the rim centered on the (no-longer symmetrical) axle, but probably no more than most road wheels are.

edit: changed the numbers slightly to fix my math. 140mm instead of my previously stated 145mm should work perfectly.