That veto right would only apply to the process for a formal amendment to the Constitution. I don't think in this context the Quebec government could try to exercise that veto. It could have a reference question on whether or not there was a convention that required provincial consent for a change like this. We have precedent from the patriation period, where the Supreme Court was quite willing to discuss the content of constitutional conventions relating to federal institutions in the Constitution.
On May 7th, 2008. See this statement in context.