Interesting request. Hopefully, Bob will take pity on you. I thought of a terribly kludgey method though - using plugouts.

afni -DAFNI_RECENTER_VIEWING=Y -yesplugouts

That tells AFNI to recenter the AFNI viewer with Then each time you want to move the montage back to the center of the dataset, you can do this:

plugout_drive -com "SWITCH_UNDERLAY mydset.nii.gz" -quit

or alias that command with something like this:
alias cds 'plugout_drive -com "SWITCH_UNDERLAY mydset.nii.gz" -quit

Then typing "cds" will drive afni to center that dataset, including montages. If you want to go to another coordinate, you could do that with the plugout command
"SET_DICOM_XYZ".