I've had good luck with feeding DOS apps UNC names (\\system\file) in the past, if you can make the file being read a UNC filename then host it on a separate box.

This "separate box" being a Unix machine using Samba (or some other file sharing system). The "file" can then be a unix named pipe as mentioned in previous responses, or you can hack samba directly to run programs to generate I/O to fill this "file" as it's being served out.

Well, I mean as long as we're talking insane ideas....