Can't chainload from Grub2 on usb-key

As I mentioned in my last post, I installed LinuxMint11 to a 16GB usb-key with Grub2 in the mbr and it worked very well, other than being noticeably slower than would be expected for operation from a HDD (or ssd).
Just for curiosity, I created some further partitions on the key and then installed Ubuntu 11.04 (/ and /home) as well as Haiku R1A3 (all of these three partitions were logicals.
Running "update-grub" in LinuxMint picked up LM and Ubuntu on the key as well as Ubuntu and WindowsXP on the Dell E520 HDD.
I then added an appropriate stanza for Haiku (on the key) to the /boot/grub/grub.cfg file on the key.
So, what booted and what didn't?
LinuxMint on the key booted perfectly but nothing else that was installed on the usb-key (Ubuntu or Haiku).
From the Dell HDD, Ubuntu booted fine but WindowsXP did not.
So, neither of the chainloaded OSes (Haiku on the key and WindowsXP on the HD) booted.
Most surprisingly neither did Ubuntu installed on the key.
So, it seems that Grub2 in the mbr of the usb flash drive only handles Linux OSes associated with the mbr of either the flsh drive or the mbr on the HDD.
I have no idea at all why but this is my tentative conclusion from a brief period of testing.

I'd like to try some other bootloaders on this key but it may be a challenge to install them. I'd like to start with Haiku's BootManager (formerly known as bootman)