Our solution for the same problem was to just have different subdomains for each cert signer (and make sure we ship the right base URL for each manufacturer's a

Our solution for the same problem was to just have different subdomains for each cert signer (and make sure we ship the right base URL for each manufacturer's app), so we didn't need to do any clever device-sniffing at the SSL termination point. I think rolling our own encryption sounds much scarier, but equally we weren't running at Netflix scale.