Port forward (NAT) based on domain/URL
The Time Capsule cannot port forward based on incoming domain request. It is not aware of the domain requested, only the port you are attempting to access. You need software, like properly-configured Apache, to handle multiple virtual domains within a network. I am not knowledgeable enough of the topic to advise you specifics, but it would involve handing off DNS and/or routing duties to a server (such as the Pi or OS X Server) and putting the Time Capsule in bridge mode.

A better way to do this is to hit the Raspberry Pi at another public port, such as example.com:81. The Time Capsule can handle this with ease.