CentOS - May 2007 - Crazy idea for a portable CentOS installation

I've got CentOS up and running on my hp pavilion laptop.  There wasn't
enough space on the internal hard drive to shrink the NTFS partition
and install CentOS there, and I didn't want to go through the exercise
of copying everything to a new larger internal drive until I knew 5
was going to work, so it's installed on an external USB drive --
except for /boot which had to be on the internal drive in order for
grub to properly dual-boot XP and CentOS.

Now the thought occurs to me:  Is there some way I can easily carry
this external drive to another machine, plug it in, and boot up the
same CentOS installation?  There are two constraints: (1) There is no
/boot on this drive, so I have to re-create that, and (2) I don't want
to have the /etc configuration that works with my laptop destroyed by
whatever kudzu finds on any other machine I might plug in to, so I
need a separate /etc.

Which leads to my question:  Can I somehow:
* copy /boot and /etc onto separate partitions on either this USB
drive or a thumb drive [let's say a thumb drive for clarity in the
rest of this paragraph], and
* re-run grub to install a boot loader on the thumb drive, such that
* the end result is that when I boot the laptop from the internal disk
with the CentOS USB plugged in, I get exactly what I have now, but
* when I boot from the thumb I get the root from the CentOS USB with
the thumb /boot and /etc mounted over it?

Kudzu could thus do whatever it needs to for the current hardware in
the thumb /etc, without changing the original /etc, but everything
else would be exactly as it is when I boot the laptop.

Is there any chance at all this will work?  I assume to change the
boot device and reassign the /boot and /etc mount points I'll have to
boot from the rescue CD.  I've never done a grub reinstall by hand
before (always used lilo in the past) so I'd appreciate pointers to
the specifics of doing that for CentOS 5.