Blondie mentions in B.C., Curtis and in Cathy.
I suspect that's about the end of the mentions in other strips. If so, it's a bit puzzling why some strips participated and so many — including ones depicted in Blondie — did not. There was one Blondie strip (this one) featuring Frank and Ernest but I didn't see any Blondie mention in that strip. Dilbert turned up in Blondie but not vice-versa. Some of the press reports said that Zippy the Pinhead would be there but they were apparently wrong. Even a lot of strips from the same syndicate, King Features, didn't opt in although a few more will appear in Blondie before the week is out. Given that Blondie is one of the most widely-circulated strips in the world, you'd think that a lot of cartoonists would see it as a great opportunity to get their characters seen on comic pages that don't normally include them.