I don't think that something has to appear frequently in order to be a running gag. Even if Excalibur not making too many appearance in the story itself, Ohkubo is still using him as a running gag in other ways. Like that time that he put Excalibur on the cover of volume 14 instead of Kid/Liz/Patti like everyone was expecting. And all of the Excalibur merchandise that Ohkubo has combined forces with GanGan to troll the fandom with. ;)

Oh god, please tell me we'll get to see Noah make the Excalibur face. xDDD

Oh, that would be beautiful!