Thank you for the suggestion! However, Scrivener has to provide the possibility to open/edit external files, then DEVONthink should automatically recognize this and add Scrivener to the Open With menu.
Technically, Scrivener doesn’t open RTF files, it opens Scrivener projects (.scriv files). To access an RTF file from Scrivener, you first need to import it into a particular project.
The Scrivener text engine is based on TextEdit, so that would probably be the closest alternative if you just want to edit a single file.