LifeSize DVI Input from multiple devices with in-line scaler
I've found an answer to my specific problem that's a little ugly, but might be of some use.

The basic premise is to ensure that the first device in the chain does not support (or is capable of disabling support for) HDCP. Macs and iOS devices are opportunistic when it comes to HDCP; they'll always attempt to negotiate it, but will fall back to unencrypted when there's no protected content playing. 

The Kramer VP-424 scaler has a setting "HDCP on input" that will, when disabled, prevent that negotiation. By putting one of these inline between the input and the switch, HDCP never starts and the whole chain is unencrypted:

Mac -> VP-424 (HDCP Disabled, 1080p output) ->VS-84H Switch -> VP-424 (HDCP Disabled, 720 Output) -> LifeSize Codec.

The scaling doesn't have any noticeable effect on image quality or unduly effect sync times, which is a bonus. I still get 1080p out to the local projector and downscaled 720p to the codec. 

As I say, it's neither pretty, nor cheap; but it works.