Oops, it seems I did not read the manual carefully enough ... The answer is actually there. So in case others need to find the same info, for future reference, one basically needs to invoke "\set TabStaff.stringTunings" with the appropriate pitch mods.
At this point, I guess using Noteedit alone is not sufficient, since it does not support all Lilypond more advanced features, including tablature. But I find myself more productive by first entering the notes in Noteedit; and then tweak the exported Lilypond output to include fingering and string info.
Also, does anyone know of a conversion tool for lilypond->xml? I guess because Noteedit can export Lilypond, but can only import xml. That means once exported to lilypond, Noteedit is useless for further editing. I know that "xml2ly" is available, but can't find an appropriate inverse counterpart.
Anyway, please share your experience and tips/tricks for using Noteedit/Lilypond for classical guitar. I'd really like to see how the process could be less painful and tedious
Of course, the nice final output produced by Lilypond should be worth the effort ...