I have noticed some things when I was trying a bit around.
When I first attempted to boot my MBP from rEFIt with the Vista Install DVD inserted and my new 500GB HDD attached to the ExpressCard there was a DOS-like screen flashing up that said:
Code:
Sil 3132 SATALink BIOS Version 7.2.30 Copyright (C) 1997 - 2005 Silicon Image, Inc
0 ST3500630AS 468GB
Then the Vista installation routine began and it ended where I had to choose the installation target. That was where only the internal HDD of my MBP showed up in the list of available HDDs with the message that "Vista cannot be installed on GPT disks".
That is very interesting indeed, as I didn't really care about the flashing BIOS-screen while booting.
Actually I was just paying attention to what happened when I hooked up my "old" 320GB Windows HDD (relic from my PC I sold before getting my MBP with Windows XP installed on it). First I tried booting it with rEFIt via the USB interface of my external case, showing up all the available partitions of the drive but plus an extra icon on the side of the others saying "Boot Legacy OS from HD". Note that the ExpressCard was still attached to the MBP, though the cable was not connected to the HDD. Choosing this option didn't result in anything as did trying to boot from the NTFS partitions, because this resulted in the (expected - because well-known) error message complaining about the lack of Legacy OS booting capabilities.
OK, then I attached the very same disk to the ExpressCard and the eSATA interface of my HDD enclosure and fired my MBP up again.
There I had only two icons appearing that were the "Boot Mac OS" and the "Boot Legacy OS from HD" icon. So I chose the latter. Again, the BIOS-like screen flashed up, this time with the matching HDD information, the screen flashes again and then it ends with a "NTLDR is missing - Press Ctrl+Alt+Del to restart" message.
Maybe there is nothing special about what I was observing with the BIOS screens, but I think this behavior is quite different from what you have experienced.
Anyway I still have to solve the problem that the 500GB external HDD doesn't show up in the Vista installation and I would like to investigate why my existing XP installation doesn't start but quits with the "NTLDR is missing" message.
So here is what I'm about to do:
Install XP via BootCamp on my MBP, flash the ExpressCard to the latest BIOS (7.4.05).
Then I would just have to find a way to install/copy the ExpressCard drivers on XP like you did on Vista and try to boot off the external drive (though this is the difficult part where I still have to find a way to get the drivers on my already existing XP installation).
I also have to sort out the "NTLDR is missing" problem.
Now I can think of two ways to make the thing work:
1. copy over/clone the BootCamp XP installation with installed ExpressCard drivers to my external 500GB disk (with or without OS X)
2. once installed, booting into the BootCamp XP installation with the other drive containing my old XP installations attached to the MBP and manually copying the drivers to this old XP installation as well as editing whatever is needed to get rid of the "NTLDR is missing" error on boot-up from this disk.
Once this runs, I could delete the BootCamp partition from my MBP and boot right off my XP installation. The same applies to Vista. Maybe I will also try the same steps with BootCamp and Vista.
Well, that is what I can imagine right now to make everything work as desired, though I will still have to try the GRUB method with the files you provided.
Gosh, this whole thing didn't let me sleep, I actually woke up to write all this down (after going to bed at like 05:00AM, just thinking about how to solve the thing, impossible to fall asleep and still sitting in front of the MBP at 10:03 - I must be insane!)
I will try and get some sleep right now and try out my theories later this day (some time in the afternoon I guess...

)
Excuse the poor quality of the attached pictures, expecially the one with the flash.