Definitely NO on the letter sign. I would go so far as to say don't even think about it. That is because the letter sign is used for a letter(s) that means a letter. THIS letter is part of a word. The special symbols list tells the reader exactly what the symbols mean.
I used the latest edition of World Braille Usage and the NBA Interim Manual for Foreign Language Braille Transcription.
Please let me know if you need any further information.