There are ways, but none "simple" or without "strings attached".
One possible way would be grub4dos and a "Vista boot floppy" (BIOS only, it would need to be checked if "portable" to UEFI/GRUB2), another one could be to overwrite/exchange the BCD "on the fly" (not really recommended).
The alternative (limited to some versions of BOOTMGR, no idea if BOOTMGR.EFI can be similarly modified) is a "hacked" BOOTMGR, pointing to (say) BC1, BC2, etc (still an intermediate reloading via grub4ods or GRUB2 will be needed).
How many of such entries (entries requiring strictly to be in the \boot\BCD) do you actually have?
Can you list them? (maybe some of them can be booted differently)