Alrighty! I am not quite sure how to upload to The Zone as of yet, so I uploaded it to Media Fire.
Included is the example WAD, MBF for DOSbox, and the MBF source.
I included MBF because that is the first source port to feature the Sky Transfer linedef.
As you will be able to see in the example, the first room you walk into has the standard DOOM II sky, but the second room has a different sky all together. It's really easy to pull off in one's maps, though I'm not sure how easy it will be to implement in your source port, especially where system resources are concerned.
Thank you for having a look!