@JohnHa: “… is an extremely useful feature for those who collect all PDFs of LO .odt documents in a separate folder.”
Doubtlessly. However, the OP wanted it extremely useful the way he likes it - for probably good reasons.
I myself, on the other hand, virtually never use the ‘directly’ icons at all, but prefer to spend the few seconds I may need in addition to quit a dialog. In my feeling it helps a bit to avoid waste of much more time in the few cases I hit the wrong icon, or a case requires different handling, while the next case may require the original defauilt again.
Concerning the OQ I sketched a solution independent of dialog styles or in another obscure way of ticklish things probably subject to changes without notion.
If interested you may study this attached example.
===Edit1 2019-01-06 12:00 CET===
Two amendments:
-1- The macro contained in the attachment is inappropriately assigned to the button event ‘Mouse button pressed’. It should be assigned to ‘Execute action’.
-2- To prepare for actual use, you need to move the macro to a module of your local standard Library. You also cannot reasonably assign it to a button conveniently then, but have to use some additional toolbar control (icon/text) or additional menu entry to which you can assign the macro. Use >Tools>Options>Customise
for this.