Are hybrid drives working well on Linux yet? Last I checked, support for hybrid SSDs was still in its infancy.
If this system is using an Ethernet connection, just get a Linux or *BSD box running with bridged Ethernet interfaces or pay for a decent smart switch. Heck, you could probably do it in Windows -- that supports bridged interfaces too.
Simply disable the interface connected to the device you want to protect whenever you don't want outside access. With a Linux/*BSD box, this could be accomplished with simple scripts. You'd probably have to write up a simple manual procedure to do it with a switch or Windows box.