Here are some ideas that somebody with technical ability and time on their hands might like to investigate, but comes under the category "too fiddly to be worth the bother" unless you are up for a challenge (I think idea 2 has the best potential):
1. Pre-stuff the TSR buffer and see if you can fool the 'Fox into playing the content.
2. Initiate media file playback via WebIF by creating a symbolic link to the desired file with the link in a predictable place in the media list tree (eg inside folder name = " " in root My Video), and launch it with an ir macro. This might be easiest on HDR-FOX using a dedicated virtual USB drive, as long as the drive itself could be named so that it appears in a predictable place in the drive list.
It might be possible to reset the My Video navigation during the macro by accessing the USB list and then returning to the HDD list (but this takes an indeterminate length of time). A solution using the USB list would be necessary for it to be compatible for HD-FOX users.
This idea can be "played with" without re-coding the WebIF: symbolic links can be created on the command line, and ir macros created through existing WebIF facilities. If a reliable solution is forthcoming, we can consider rolling it into WebIF for everyone.
3. Use a DLNA server (eg on a PC) to create an open-ended stream, accessed via the 'Fox's DLNA client. You would control what was fed into the channel at the PC end, but it would then be "rendered" by the 'Fox.
4. Use a custom portal to provide portal access to our own locally-hosted Internet radio or YouTube stream, and feed that from media files within the home network or even on the local disk.