Perhaps the legalities of staff travel removal for strikers should be heard in court to give a definitive answer once and for all.
I think think the union would be on dodgy grounds. The strikers did not have their staff travel removed because they were on strike, they had their staff travel removed because they were told it was going to be removed if they went on strike. I think this thin line of being told before hand would be in the favour of the airline (like other poster's caveats, I have no legal back ground so this is just an opinion) On that basis it would not be discriminatory.
As for SOSR, it would be in the airlines' interest that this was not instigated, however if this latest offer is rejected then a 90 day notice of new contracts may be applied. (Using the contract already signed by the non union cabin crew)