Just thinking laterally for a mo.....I have been VDB by bmi quite a few times. Not that it says so on the the instructions accompanying the voucher you get, but it can be redeemed against codeshares - e.g. I am using vouchers against a family trip, including IAD - MCO (with UA).
As all of the big US airlines have codeshare agreements throughout the world, might it be possible to use vouchers on these, assuming they work the same way as bmi admittedly?