Another way to ask this .... is there anything in the Litmus Analytics toolbox to identify the split? Or does everything in the Email Clients section roll up into the "Via Gmail's Image Cache" bucket?
Some of my colleagues at MapMyFitness (one developer and someone on the business side) thew AppRedirect.com together one weekend. The idea was initially to solve our internal business case need to drive in-app engagement, but they felt others could benefit from the service and put it out there (albeit without any promotion). I think they abandoned the project pretty quickly after exposing the service, but it does get the job done with the important exception you mention above. This is a pretty new area but growing in importance so I'm eager to hear what others are doing. Also going to check out Deeplink.me.