It won't be the undoing of the argument as the argument is the breach of the intent of Schedule K. Cityflyer is permitted under the Schedule K agreement. The argument is not about Open Skies or different terms and conditions.
If BA decide to replace the RJs with 318s at LCY then the Citiflyer pilots cannot be laid off as BA has to offer them similar employment elsewhere in the BA group, which to all intents and purposes means mainline. I'm sure BA pilots would offer moral support and some behind the scenes advice from the BACC, but direct industrial support would be classed as secondary action and as such is illegal, just as it woud be if you offered direct support to the BACC in this dispute.
TLP - when the leases come to an end Cityflyer can either wind up the operation or switch to RJ85s which are permitted under Scope. Given that they are already moving to find 85's I'd say the latter option is by far the most likely.