in electric and steel string guitar The Left hand thumb is sometimes used to fret the 6th string (J. Hendrix)
would be great if added to the wonderful guitar implementation
Please excuse my ignorance, but can you attach a picture so I can visualise what you’re talking about? Thanks.
It’s written as a ‘T’.
So you would have T, 1, 2, 3, 4
In this example, and in most that I think that I have seen, it is in editions where the ‘T’ indication exists in TAB and would therefor be fine with a text input. TAB never really supplies fingering other than this tip. This is a Cherry Lane Music Company Edition.
Here is one where the T exists on the manuscript, where the F is played on the 6th string with the thumb
Sorry to hijack the thread, just bustin’ in for expedience 
+1 for this. Just found myself needing this yesterday (transcribing a Kenny Burrell solo).
Just to follow up on this thread: it will be possible to create a ‘t’ for left-hand thumb via the Shift+F popover in a future Dorico 3.x update. This won’t appear on tab (where you can, I think, easily add it as a playing technique that is set to appear only on tablature) but it will appear in the appropriate place on the notation staff.