Don't even need to go the C# Major route either, writing directly in C# Minor you're still likely to want to use B# from the harmonic minor. Which IMO is common enough.You are correct that sextuple flat never occurs in music. Otherwise you are wrong. Double sharps and double flats occur with some regularity throughout tonal music. Obviously, they are the rarest kind of accidental, but they occur with significant regularity.
A very early example of B sharp is the C sharp major prelude from Bach's well tempered clavier. Wtc. Here's a score showing the b sharp right in the key signature. Imslp. C-sharp major is not nearly as uncommon as you would think because music in C sharp minor may make use of C sharp major as the parallel major key. C sharp minor is incredibly common because it only has four sharps in the Key signature. Even C sharp minor makes use of b sharp for the dominant chord. If you're playing music in G sharp minor, the dominant chord will be D sharp major, which will have an F double sharp in it.
Edit: I decided to reply before I read you mentioned using the dominant, which is essentially the reason for B# in the harmonic minor, ignore me