Not, it seems, a widely used technique these days. All the same, I assumed Dorico would implement it. Here’s the Wikipedia entry’s examples:
I think I’d favour the first two – it looks like something that could be implemented in the Properties panel.
The second is easy: just hide the noteheads.
Diamond and X noteheads also very easy, just by changing the Notehead set.
The first is more difficult – beams at the notehead end, and flags at the other! 
You might be able to do it with two voices…
Hmm, no that doesn’t really work.
Thanks. Simplest is best; how do I hide noteheads?
Ah! Engraving…
Thanks Lillie & Ben
@Marcabru
I’m just curious:
Would you suppress these notes for (computer) playback or use some pitched instrument to simulate the pitches in spite of the normal sounds of sprechstimme?
My playback doesn’t realize the notehead is missing. And no, life’s too short 
You misunderstood me: I don’t want to suppress it. And there aren’t the kind of VSTs that could simulate human vocals… and it’s just one note.
It’s worth noting that Sprechgesang, a kind of recitative between song and speech, as in Schoenberg’s Pierrot lunaire (example 3) or Berg’s Wozzeck, considers the pitches for speech. This is indicative, for the sake of expression, but it’s important. So it’s not a good idea in these cases to remove the noteheads.
Your first example is not Sprechstimme, it directs the reciter to whisper without using any pitch.
Direct translation: toneless whispering (tonlos geflüstert)
