[itsme]

@ jeffn1: in your first post you mentioned "Hacker's Keyboard", I also use this on my Android devices.
I managed to add the sharp and flat symbol to the "Suggested Punctuation" bar of Hacker's Keyboard: I copied sharps_flats.pro to my tablet, renamed it to sharps_flats.txt and opened it in a texteditor. When I start editing, Hacker's Keyboard shows up. I copied the flat symbol to the clipboard, opened Hacker's Keyboard - Settings - Suggested Punctuation and pasted the flat symbol to the list. Same with the sharp - voila, you can insert the symbols from here.
       

What I did not understand by now: this "Suggested Punctuation" bar is not always shown. It pops up in certain situations while editing a text file. Do you know which action triggers the "Suggested Punctuation" to show up?

Just in case (it was missing in sharps_flats.pro)
natural: Unicode Character 'MUSIC NATURAL SIGN' (U+266E) 0x266e
I noticed that the natural sign is missing in several fonts that contain both flat and sharp - pretty strange