Another thought: since you've got a half-link, you can adjust the chain length in 1/2" steps. That means you can adjust the rear-axle-to-BB distance in 1/4" steps, and consequently your rear axle must be within 1/8" of a suitable position - possibly less. Can you grind that much out of the drop-out slot without fouling the axle fixings? The idea is not to widen the whole dropout-slot, but just to grind away a little material where the axle sits so that the slot is "L" shaped. With care a simple circular file might do the trick.