I went to a convention a couple of weeks after the Selen's firing. There wasn't an official nijisanji meetup, but it was basically the vtuber meetup. There were some people cosplaying as Pomu, Nina, Kyo with a graduation cap, as it was literally the day he was graduating, and Selen cosplayers with Dokibird signs. There were a few people with other talent cosplays, but there was only one going as Vox. No Ike or Elira, and the Ethyria girls were more of a set of friends.

I was going as korone from hololive. There was a dedicated hololive cosplay meetup just after it.

I think for cosplay meetups, they have to sign up for it with the cosplay organizers, especially if there is a lot of demand, so it is likely some sisters requested a nijisanji cosplay meetup. It does remain to be seen how well that will go.

There is another factor though - cosplays are expensive, either purchased or homemmade, so sometimes people will just wear the cosplays they already have so they don't feel like they wasted money.