Calendar free/busy interop with Google Apps Calendar?

[Rich Graves]

We're interested in exchanging (so to speak) free/busy lists with another local organization that uses Google Apps.
Has anyone tried mating the "Google Apps Calendar interop" feature with Zimbra's "Exchange free/busy interop"? Since both products are supposed to interop with Exchange, shouldn't they be able to interop directly with each other?
(ZCS 7.2.2 NETWORK)