An interesting question. Unfortunately I don't think it's possible to restrict/secure stuff that's exposed via the DLNA streaming service (the "share for media devices" service).
I've never come across a DLNA server that supports such a restriction - to be honest the whole point of DLNA is to advertise on the network and make stuff available for streaming to anything that can talk to the server. I did find one similar question posed on another forum back in 2009, doesn't look like any answer was offered.
I suspect the only solution is NOT to share the more sensitive media using DLNA, but to keep it in a Personal folder on the GF Home and access it through password protected file sharing (some media players can connect to servers in this way, eg. the Viewsonic VMP73 box that I use)
