I’m glad you are asking this question about Lewis dot structures in a UEB Math/Science transcription. I agree this is a challenging situation because we do not find a lot of guidance in the UEB rule books.

And yeah, Chemical Notation Using the Nemeth Braille Code 2023 §2.5.4 provides that lovely two-cell symbol for “three pairs of regular electron dots.”

I think the UEB transcription you suggest for three pairs of regular electron dots, though unweildy, is the best we can do at this point.

⠠⠓⠨⠐⠲⠠⠉⠨⠐⠲⠨⠐⠲⠨⠐⠲⠠⠝⠨⠐⠲

That is “H double dot C three double dots N double dot”