Change of time signature: how to notate the tempo guide?

Add a tempo text (click note, press Alt+T) then press F2 to display the Special Characters palette. You'll find everything you need there. Hint: the space right before the copyright sign is designed to be the right width to place between the note symbol and the dot symbol. A regular space would be too wide.

I can't seem to find a dedicated way to do this, but I can suggest a work-around that you can use.

Type F9 to open the palette list, then click on Tempo. Click on the note in the staff to which you want to anchor the metronome marking, then double-click on the metronome mark in the palette which contains the note value you want first. Now do the same thing again, this time using the second note value you want, and anchor it to the next note on the staff. You will now have two metronome markings side by side, each with a different note value =80.

Double-click each of these in turn to enter 'edit' mode, then remove the numbers and the extraneous equal sign, leaving only the two notes and one equal sign between them.

Click in a blank part of the score to exit edit mode, then type F8 to bring up the inspector. You can use the inspector to move those elements toward each other. Click on an element to select it, then adjust the horizontal offset until it has moved to where you want it.

Finally, uncheck the 'Follow Text' box in the inspector for each of those metronome marks so that you can set the actual playback tempo manually.

You can adjust the actual size of the notes the same way you would reformat any staff text, using the edit bar at the bottom of the screen that shows up when you double click on them.

Once you have created this crotchet = dotted crotchet, you can [Ctrl][Shift]drag it to a custom palette for later use.