Hi 
I'm not sure how to open as read-only myself but i'd guess a right-click
either per file or on a group of selected files prolly has something.
Select "Open File" within Libre Office: in the dialogue box, in the
lower left hand corner is a clue and yes, it does function with multiple
selected files.
I think chmod might help change which user "owns" the files - or put them
on a volume/drive/folder that is mounted read-only - but that all seems a
bit extreme.
It's more than a bit extreme, but changing the permissions on the
reference files would work, with possible side-effects. i.e. Libre
Office would probably try to auto-save and might regularly bleat about
its inability to do so.
You could change the time-delay or completely eliminate the auto-save
feature. Such options used to be in;
Tools - Options - ...
or something like that and prolly still are.
Not a good idea if one wanted to auto-save the "live" document, however
depress F1 and search for "auto" in Help.