I confirm the customization of the binding menu background is not an implemented feature.
The following manual procedure can be applied to attain it.
The background picture for the binding menu page is stored on each Bare Metal server in file:
You can replace this file with any 800x600 resolution picture you want to use (renaming it to tpmfosdbg.png). This will replace the standard binding menu background, for both BIOS and UEFI boots.
Please note that a BM server service restart will restore the original picture, so it will have to be replaced again.
Please also note that this will not change the screen that is presented on the target screen during the deployment,
Hope it helps