The usual approach is to go to Settings->Outputs, and select Output 2, 3, or 4 (1 is reserved for streaming/recording), and select the input for your camera. Then in the instant replay settings, instead of specifying the input directly, select output 2, 3, or 4 as defined before. Unfortunately the number of outputs is limited. so if you have more than 3 cameras that you want to do this with, you may be out of luck. In that case, you'll need to put in a feature request. :)