Is there a way to show a tie (or slur) that trails from a note so as to indicate that note (typically an open string on string instruments) should be allowed to ring? I have not managed to find a way of inserting a tie or a slur without a second note at the right-hand end but I could be being dim!
Not yet, as far as I know, but I think it is planned for the future.
You are not being dim, John: this is not yet properly possible. Various fakes are possible, depending on your degree of determination and your threshold for an acceptable result. You could add a slur to the previous two notes and then move it to the right in Engrave mode, for example, or you could copy and paste characters from e.g. here into Shift+X text items in the score and move them around in Engrave mode.
You could also tie to a new [extra] note (that you don’t want) and then change the opacity of that particular fake note to 0%. I’ve used this fake to more precisely place glissando lines where I want them when dealing with chords. (In this case it would probably have to be a different pitch—a sharp or a flat of whatever you’ve already got—so that it isn’t absorbed into the tie chain.) The extra note merely serves as the anchor point for the line but doesn’t have to be shown itself.