Can you not manually enter the path to be excluded (copy and paste using explorer) rather than use the FSS UI exclusion interface. I can't recall if this is the correct term but avast will recognise the UNC format path to the network locations.
If you want to exclude a folder and all its content/sub-folders there must be <path>\* at the end. If you want to exclude a particular file type <path>\*.v2i (example of my drive image backup files), etc. Care has to be taken when using wildcards ? (single character) and * (multi character) or you could leave big holes in security.
Sorry I can't be more helpful as I don't use a network.
Edit correction of incorrect term.