Upnp is just a protocol a client can use to discover other network devices.
Try to mount a remote nfs share -say- into /mnt/nfs-nas.
then browse it with xt7 via "browse" tab, i don't think it will be slow at all.
If it is, then add the whole folder to the watched directories into "general options" -> library -> watched directories and issue a library syncronization.
I don't think it would be *so* difficult to add such functionality but it doesn't worth the pain, it is just useless other than limitating.
If you really want to use dlna, take a look here:http://djmount.sourceforge.net/
djmount is a UPnP AV client. It mounts as a Linux filesystem the media content of compatible UPnP AV devices.
Djmount discovers automatically all UPnP AV Media Servers on the network, and make the content available in a directory tree. All shared files (e.g. Audio or Video files) are directly visible and can be played using your favorite media player.
Hope it helps.