The only way you can boot one Mac off another is if you put one of the Macs into Firewire target disc mode.
That won't let you select disk images to mount before booting though, so you would only be able to boot your MBP using the MB's regular OS X install (which will probably not work as rman mentioned).
You could repartition your MB's harddrive to have one boot partition and one partition that has a full image backup of your MBP's drive.
Then you would be able to select both images in target disk mode and boot off the image you created of your MBP's drive.
However, it seems a lot easier just to get a proper external backup drive.
Interesting. You might be on to something. I am not in love with external hard drives for a very simple reason. I have a home network, with a couple of servers, which are not Macs. In all, I have 5 computers. Having external hard drives for all isn't a good solution because it ends up being wasted space, plus there is no redundancy. So instead I have a central backup server with RAID and a lot of space. So I can keep all my computers backed up in one place.
The downside to this with my main computer (MBP) is that if my hard drive dies, I can't use it until I replace m hard drive (because I don't have an external drive). My planned alternative would be to get an external drive just to use for this situation. But I have a MacBook as well. My MB is a secondary computer that I use when I travel or sometimes when I want something small and light. I don't need it all the time and so it's kind of an extra machine. So I was thinking an alternative to buying an extra hard drive would be to just use my extra computer.
Let me say this, I have no problems formatting my MacBook and imaging it with my SuperDuper clone of my MBP, then using it in Target Disk Mode to boot from. I can wipe my MacBook if I need to use it for this, but I was wondering if this is actually possible.