Registering additional masks in fMRIPrep

I am wondering if it is possible to include pons in the linear registration part (native fMRI to native T1 registration) with step (FLIRT with the BBR cost function and with the pons as weighing volume) in the current fMRIPrep version?

Thanks in advance!

The white matter mask derived from the FAST segmentation is used in the BBR step, but there is no way to include a custom mask at present.