Firstly the Governor General and Leuitenant Governors were only really relevent in the 1800s and early 1900s. The elected officials command about the Governors now.
The last withholding of Royal Assent occurred in the 1930s. In 1936, the Lieutenant Governor of Alberta, John Bowen, withheld Assent on three bills, including one that would have restricted freedom of speech and was later declared unconstitutional by the Supreme Court of Canada and then by Her Majesty in Council. The Lieutenant Governor performed such an action to maintain order and to restrict the overreaching Government: I think that a modern Lieutenant Governor should be able to perform such a function as well.