Here's how it works.
WP7 has whatever NAND it uses (512 or 1gb I think) and it actually incorporates the SD card as part of it's system storage, basically like a Raid Zero, so the SD and NAND are actually connected so that they are one and the same. If you remove your SD card and turn on your phone it will hard reset and show "low storage" and won't do much.
AFAIK there is no way to mount it as external storage for this reason, it fully utilizes the "secure" part of SD and even on a partition manager it comes up as "unknown" you cannot separate it or reformat any new part of it.
On the HD2 you can actually take a 4gb card, hard-reset so it uses the 4gb partition, copy that partition to a 16gb card, and use the other 11.5gb to do whatever you want with. I am assuming this would work on any other WP7 device, but the HD2 is the only one that can mount external storage via the bootloader/CWM/Android on SD, now if you could mount external storage thru the bootloader you'd be good, but not till you get a custom flashed bootloader, which probably won't be soon.
So the answer is
AFAIK you cannot mount external storage no matter what, and you could trick the SD card to hold like a 4gb WP7 partition and an 11.5 FAT32, but you'd have no way to access it from the phone.