I'm looking at using an A2 flight controller on a new S900, but my colleagues would like to use the stock Phantom 3 remote controller we have already, with it's integrated app + video link. Is this possible? Would I need additional equipment to transmit video from the S900 to the controller/app (iOSD?)
Thanks

Unfortunately that configuation is not feasible. Your best bet is buying the Lightbridge 2 system, which does have a similar configuation RC. That will allow for you to use the Go app for FPV.