We probably don’t want that sort of off-the-hook communication between snaps as it becomes pretty much impossible to mediate. Who else will be opening that file besides libreoffice and mozilla?
The proper way to do that is to ask the application registered to open files of that type to have a look at that file, and only after the user has approved that interaction.
There’s some relevant discussion about this at the bottom of the xdg-open improvements topic.
Meanwhile, sharing via somewhere in $HOME may be the best way to go indeed.