does Dorico provide these 8ths?
I must apologise for my last post - wrong headline…
You can certainly change the noteheads easily! Right-click the selected notes to access the contextual menu.
You don’t need a workaround for this. There’s an option at Engraving Options > Tremolos > Multi-note tremolos.
Is it a tremolo, or shorthand for notating eighths?
Surely it’s either or both: a slow, measured tremolo that equates to eighth notes.
As written in the OP’s example, it’s a shorthand equating to regular 8ths.
@Piano_Vink : this is easily accomplished natively in Dorico. Enter the four notes as quarters, select them, call up the repeats popover (Shift-R), type in 12 Enter (1 for eighths, 2 to connect them by twos). You’ll probably have to flip the first and third notes. If you want the notation as in your example, you’ll also need to make sure that your Engraving Options/Tremolos/Multi-note Tremolos are set to allow all lines to join the stems or the outermost lines to join the stems.