I don't know if this is possible, but it would be nice if "PC Directory" was (optionally) not present in the tree presented to devices. For example, I have the following showing up:
Everything in PC Directory is already accessible in the other containers and it's less "user-friendly" for it to show up on the device. (It should still be present in the Web UI.)
I intend to look into this myself, but wanted to add it here for tracking purposes - it'll take me a bit of time to get up-to-speed with how uPnP and (in particular) MediaTomb work.