This is basically copy of bug #7449 from OOo bugzilla , reported almost decade ago (!).
LO gives ability to modify toolbars and menus, but there is no way to modify context menus. There could at least be some text file around and/or USO service to register in macros, as doing GUI may be quite difficult.
Said bug says that there has been some work done to implement it, so it may be good place to start.
Thanks for reporting (last time I saw also some enhancement request passing buy from you :-), thanks multiple times! :D)
Mark it as NEW and add that link you provided to the 'See Also' field. Also can't find any duplicates so far.
*** This bug has been marked as a duplicate of bug 93837 ***