It's a limitation you see on some content on most DLNA "servers." Most thought it would do that but you are talking about a system designed in committee down to the lowest common denominators.
Your question has been asked many times and there is no sign that a DLNA II is in the works or that makers will offer an update for this.
I'd research and try other DLNA servers.
I just bought a PS51D8000 plasma and have been trying to watch my videos through samsung allshare(streaming from my PC, wireless).
Everything works fine except fast forwarding or rewinding does not seem to be possible.
The videofiles I have been using are .avi and .mkv
Are these formats not supported? If not are they going to be supported in the future(through an update)? Are there any other file formats with supported ff/rewind?