How would I go about creating a virtual drive with which I can programmatically and dynamically change the contents?

For instance, program A starts running and creates a virtual drive. When program B looks in the drive, it sees an error log and starts reading/processing it. In the middle of all this program A gets a signal from somewhere and decides to add to the log. I want program B to be unaware of the change and just keep on going. Program B should continue reading as if nothing happened. Program A would just report a rediculously large file size for the log and then fill it in as appropriate. Program A would fill the log with tags if program B tries to read past the last entry.

I know this is a weird request but there's really no other way to do this... I basically can't rewrite program B so I need to fool it.

How do I do this in windows? How about OSX?