Is there a notation convention for this?

[stanron]

I'm about to notate a finger style piece where, as well as picking bass notes with the thumb and melody notes with fingers, I also play two or three note chords by tapping the strings with my fingernails. Is there a notation convention to show this or is this a wheel I have to invent?

[rick-slo]

"T" often stands for a tap. You might want to insert some text at the top of the tab describing what you mean by whatever symbol you decide to use.

[stanron]

That's the sort of thing I was thinking of.

TC = tap chords lightly with fingernails

at the start of the score and then put TC above the chords.