You can address it whatever way you want it. It could be labled as an incomplete inversion chord,
Bb (b5, maj7th)/ C#
I do see where it resolves to though. Take the C# and lower it a half step, you get a C dom 13. I would say its a slash chord with C# as root that's passing.
I see. I was thinking if its arranged as Bb Db Fb A, with the minor 3rd at the bass string, some sort of inversion of Bbdim? I saw someone playing "How Insentive" and the chord progression went from Dm6 to this chord to Cm13.