MacBook – Old MacBook power adapter on new MacBook Pro

Apple's official word on the matter is:

Make sure the proper wattage adapter for your portable computer is used. Select the appropriate power adapter for your Apple portable computer. You can use a higher wattage power adapter, but you cannot use one with less wattage without potential operating issues. (here + discussion here).

So your 13" can use your 15" charger, but not vice versa.

I've never heard of it voiding a warranty (nor experienced it when we've used the wrong charger), but it's better to be safe than sorry.

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.)