In addition to the internal ram cache, how about supporting the use of any other directly connected device (flash media on usb, a dedicated hard drive, etc, etc) for cache. This cache could be pre-populated based on selected playlists and would hold all files necessary to the play list (media, cover art, anything else).
Simply supporting devices that are accessed via a LOCAL UNC path would take care of this just fine.
For the sake of performance, MC needs to be smart enough to refuse the use of slow devices, otherwise what's the point?? USB must be v2.0 speeds. Hard drives CANNOT be the system drive (C:) or the drive that MC itself is installed on. And so on. The point of the QuickPlay media is QuickPlay, not just another configuration option for the sake of variety.
As an 'extra', MC could manage a catalog of these cache devices. Think about creating and maintaining a list of the cache devices (say, thumb drives, CF/SD cards, etc) and what is on them for QuickPlay. If the library owner updates a playlist, MC would be smart enough to remind the owner that the playlist is also maintained on a cache device and "would you please insert the cache device so I can write these updates to the device".
All it would take would be a signature file unique to each cache device (NOT the volume label, winderzz can screw with that at any time). The sig file would be nothing more than a media ID key. It could even be a xero-length file with a unique hashed name. MC would read it and do what is necessary.