Not something I would use.
FWIW, I often bring text that was already created outside of Tinderbox into my notes. If need be, text can be selected in Tinderbox and dragged to another app for editing. Or use PopClip to move selected text into BBedit or TextEdit or TextSoap, modify it there, select and copy, and then flip back to Tinderbox where the text is still selected and so pasting replaces what was there. Maybe not as slick as “Open in External Editor” could be, but just as fast.