For the 'recent documents' menu selection under 'file', it would be nice to: 1) allow us to set the number of recent documents, under tools/options.
I know this is not quite what you are asking for, but in the absence of exactly what you want, you may like to try customising the Standard toolbar to display the "Load URL" command. (Click the down-arrow at the right of the toolbar and select Customize Toolbar... .) This gives a much longer list of recent files. Note that the customisation needs to be made separately for each LibreOffice component that you wish to display this, but the list itself is common to all of them.
I remember seeing some packages that allow you to define the number of "recent documents" are kept in the queue. I never seen the "permanent" one, ...
Earlier versions of (close your eyes if these are dirty words for you) Microsoft Word (at least) had a "Work" menu which allowed you to create your own list of frequently used documents - which would not evaporate until you wanted them to; later versions of Microsoft Office have dropped this idea but allow you to pin items already on the recent files menu so that they do not drop off.
Brian Barker