Time Machine from networked Mac? Is it at all possible to setup Time Machine so that it backs up to a disk image on my external Mac?

Here's my setup:


My MBA which will be backed up
MBP which is connected to the wireless network
External drive connected to the Mac which I want the data to be backed up to
MBP is running Snow Leopoard
MBA is running ML


Is this possible? I could also connect this drive to my router, but as of right now, it doesn't look like it's seeing it.
This tutorial should be able to help you:

http://www.reasonablyopinionated.com/2011/11/time-machine-backups-to-drive-on.html

I just tried it and it worked for me.  Not sure how reconnects will go after a reboot or something, but it shouldn't be that difficult to get working again.