I want to use LilyPond to write guitar tabulature, but it would be quicker for me to be able to input fret numbers and string numbers instead of note letters. How can I do this?
You can use TuxGuitar which supports entering numbers and then export it as a Lilypond file under File -> Export -> Lilypond.
Inside Lilypond itself, I can't find a way to do this. You can sort of do this by making a custom fret diagram (which uses string and fret numbers) for each note or chord and then applying that to a tab staff without using a fret diagram staff. Scroll down to the second "Selected Snippet" here to see that in action.