I hope they go with a Progression Tree skill instead of limiting us to Mirrorswords for the ability, especially since normal Howling Banshee can only use Power Swords so far. But the Mirrorswords could have a higher chance of deflecting bullets, or they could give it a Mod which could increase "Bullet Deflection".

Will Mirrorswords even be available for launch?
They did state that dual-wielding wouldn't be out before post-launch (might only be for Lightning Claws), which would make it silly to release the Mirrorswords since they always come in pairs, and since two-handed melee weapons are also post-launch the Executioner won't be available for Exarchs either, leaving only the Triskele left for the Howling Banshee Exarch at launch, which I really do not believe we will get.

But then again, if two-handed melee weapons and dual-wielding both are post launch then it will be hard to give the Exarchs a lot of options. The Dire Avenger Exarchs can't dual-wield Avenger Shuriken Catapults (at some point I thought this would be our "Stormbolter EQ"), the Striking Scorpion Exarchs will also be denied the Biting Blade and possibly the Chainsabre too (dual-wielded), even the Scorpion's Claw normally comes alongside an offhand weapon, though I guess it could be used without anything in the offhand.