I'm not sure if this has any relevance to the crossover, but one of the hosts of Supergirl Radio, Rebecca Johnson, mentioned in the latest episode of the podcast that when she recently toured the WB lot, the tour guide let it slip that he'd seen Candice Patton (Iris) on the lot, so we'll see what ultimately ends up happening.
For now, though, I'm taking Deadline at their word re: dimensional travel being involved in the crossover.
As an aside, author Andy Mangels played around with the contrast on the instagram photo of the title page that Grant Gustin posted, and was able to see through to the cast page clearly enough to read the names Italicia Ricci, Brit Morgan, Jenna Dewan Tatum, Siobahn Smythe/Silver Banshee, Livewire, and Lucy Lane, so take that for what you will.