It is possible to create an EFI application which temporarily modifies the ACPI tables (including BGRT) and then proceeds to load the actual Windows boot manager. This application must then be booted instead of the original Windows boot manager (bootmgfw.efi). BGRT uses the common BMP format (Windows 3.x BMP, or BMP3) with 24-bit or 32-bit colors.
I've created HackBGRT, which is a simple EFI application for the purpose. HackBGRT reads its configuration file, loads the specified BMP to BGRT, sets the coordinates appropriately and loads another EFI application (usually the Windows boot manager, bootmgfw.efi).
The Clover EFI bootloader can also modify ACPI tables, but it's far more complicated, and I'm not positive if you can actually change the BGRT image with it.
Intel has released some tools which apparently may be used for modifying the BGRT on some motherboards. I'm not aware of any other tools for specifically modifying BGRT.
For non-UEFI systems (or possibly for UEFI systems without BGRT), you can use the various bootres.dll hacks found on the Internet.