I get some time off of work in a couple of day, so this is one thing I will look into.. However, I don't think whether or not the media is available or on a mounted disk or not is stored as a flag in the database - and checking each db entry against actual files would take a lot of un-needed overhead. Simply put, making sure the files exist is handled by other parts of LMCE, and may be over-redundant to include such functionalitly in playlist building.
However, if there is some sort of "exists on disk" flag in the database, it may be feasible. Also, in my optinion, it would be best handled at the plugin level to fail safely if a file does not exist. This way, you can still add a file to your playlist that exists on a network share for example. When playing back, if that share is not available, then it should fail silently at the plugin level, and continue to the next playlist entry. This to me seems more flexable and a more proper way to do this.
Any thoughts on this?