It isn't a reasonable expectation to have lounges at every outpost used on an occasional/seasonal basis. And if there is no lounge then BA can't sign up for it.
On the other hand handing out a voucher which according to OP is not useable in the only food outlet is a bit sneaky.
In this era of technology surely better use could be made of splash screens when booking to warn customers of known exceptions - we don't offer a lounge at Chambery, your flight to Dubai is operated by Air Belgium etc?