Update:
G3D: No Focal Offset setting
Z-normal: Focal distance setting at 0 is max separation. Only + adjustments are allowed, which actually decrease separation – the opposite of what was asked for.
Z-Adaptive: Focal Offset setting is available in both positive and negative directions, and actually seems to work to a relatively minor degree. Unfortunately, it needs to be combined with an adjustable convergence setting. One exists but the adjustment it allows is only minor.
All this is tested with a single officially unsupported game so it is possible that the used profile is to blame.
Looking forward to testing these settings with other games in the future…