Now, I certainly don’t want to switch back to Finale, and I’ve really grown to love Dorico, but there are sometimes situations where I wish there was more freedom in editing certain elements. It’s not about whether the notation conforms to the rules of Mr. Ross or Mrs. Gould, but rather it’s important to me that I can do what I want to do. Imagine you want to capitalize the word “Table” in a text—for whatever reason, and even if it’s incorrect—then the spell checker would tell you it’s wrong, but you could still leave it in. Wouldn’t such program behavior be desirable in a notation program as well?
Hopefully, this badly needed capability will be added at some point. Until then, one could create the dot as a Playing Technique and move it wherever one likes.
The middle note consists of two overlapping voices, one undotted in a second down-stem voice with a hidden notehead, the other one dotted in a second up-stem voice, but with hidden stem only (its dot appears above the line). These middle voices get voice column index 0, the outer voices get voice column index 1. The inner voices will exhibit some superfluous rests, which you need to remove.
Officially, when you hide a notehead, nowadays the dot will (should) be hidden as well, but here, it strangely re-appears when notes are superimposed (this might be a small bug). Therefore, the note that gets its notehead hidden cannot be dotted in this instance.
Thanks to everyone who contributed. Your suggestion (John_Ruggero) to include the period as a playing technique seems to be the best idea to me for the moment.
I used a regular period (.), but the font size would need adjusting. This could be done by eye using user defined scaling in the properties area, for example, but how can I know what the correct font size should be, and where can I adjust it? I found I can right click the dot and choose edit font, but can I be sure that this change does only affect that dot? Or may this have an effect on other existing playing techniques.
Sorry, if this sounds stupid. I am still a beginner with Dorico.
Edit Font takes you to the appropriate font style or paragraph style for the item you selected. When you change that style, you change all items using that style.
In the case of playing techniques, this would be the Playing Technique Font font style, and changing the style will affect all text playing techniques.
What you should do in this case is create the playing technique as a glyph instead of as text.
In the playing techniques window, change the Type of your technique from Text to Glyph
Click the pencil icon under the preview window to open the Edit Playing Technique window
Click on the garbage can under the preview window to get rid of the text you already have there
In the Range dropdown on the right, select the Individual Notes category
If you don’t see the dot in the displayed glyphs, then you can search for “dot” to locate it
Select the dot, then click the Add Glyph button in the lower right
Click OK to close the edit window, then OK again to close the playing techniques window
(but not succesfully 
) and now with that grown knowledge I’ll go back to it an give it another try.