Oh yes, they can be assigned a drive letter alright.
Just use mountvol (it should be able, running Windows 7 + to assign a letter to the ESP partition), if for any reason that doesn't work try diskpart:
Still, normally in a GPT disk the firmware is UEFI, so in the ESP partition there will be the normal .EFI boot files.
You need to add there the BIOS boot files (i.e. BOOTMGR and \boot\BCD) and the grldr (and if neded the menu.lst, if you put the menu.lst in another partition or in a different form default path you will need to edit the embedded menu.lst in grldr to make it find the "full" menu.lst).
What the UMBR does in a nutshell is to chainload the grldr, so that as long as the grldr resides in a filesystem that grub4dos can access (i.e. not the bitlockered volume) it can be chainloaded, the ESP partition seems like the most logical choice.
I don't think you actually need two copies of grldr.
If you don't want to have the grldr and menu.lst in *any* partition, you can try using my approach that creates a (hidden) small volume in the unused space LBA 63-2047 containing all the boot files.
This volume won't be normally accessible by the booted windows, but you can mount/access it in grub4dos or use IMDISK with direct access, the only issue might be that the booted windows won't be able to mount/access the BCD.
Of course nothing prevents you from having in the hidden small partition just the grldr and menu.lst and have the BOOTMGR and \boot\BCD on the ESP volume (ot in another accessible, not bitlockered, volume.