Power Macbook Air with Early 2008 Macbook Pro
Much like the commercial versions you mentioned, what you want to do is possible (that there are commercial versions essentially proved this). 

You'll need to chop apart a MagSafe cable or figure out how to interact with Apple's airplane adapter. You'll also need to figure out how to interact with the batteries to get the correct voltage (and maybe do cell wear levelling). 

Also, I doubt the MacBook Pro batteries will last anywhere near as long or be as reliable as the commercial versions. 

(There was an original answer, but it made incorrect assumptions that were addressed in the comments. See the edit stack for the originals.)