As part of a multi-party interview with The Hollywood Reporter in which Arrow's Marc Guggenheim interviewed and was interviewed by Agent Carter showrunners Tara Butters and Michele Fazekas, Guggenheim confirmed rumors that a Supergirl crossover with The Flash and Arrow would not be out of the question when CBS launches the story of the Last Daughter of Krypton in the fall.
A reporter from THR then chimed in that it was in fact built into executive producer Greeg Berlanti's contract at CBS that such a crossover would occur, and Guggenheim didn't respond, seemingly confirming the rumor by his silence (although that might be reading into things).
Here's the pertinent piece of conversation:
Fazekas: Marc, CBS is doing Supergirl. What is the possibility of a crossover?
Guggenheim: It's possible.
THR: I actually heard it's in Greg's deal, that Supergirl can cross over with Flash and/or Arrow.
Since CBS is a co-owner with Warner Bros. of The CW, almost no one has claimed it was outside of the realm of possibility for such a crossover to take place,but if THR is right that it's actually built into the contract, one assumes it's something they're planning for.