What does this mean for Project Gutenberg Canada? How much research is needed before >they could conclude that the identity of an author is not commonly known?

Most "known" pseudonyms are either catalogued in the national union catalogue, or available through dead-tree reference sources. If searches of those two sources fail, then a letter of inquiry to one of the literary copyright collectives would probably suffice to act as a cover-your-ass. If these three lines of attack did not uncover the true identity of the author of an anonymous/pseudonymous work, then a court would probably conclude that the anon/psued term, not the life+ term, would apply, or was reasonably relied on by a third party re-using that work. This is a section, along with its predecessors, I have flagged to see if there's been any judicial comment on since the 1924 act was passed.