The surface of the custom door leaf is fixed by default, saved with the surfaces of the element it is created from. That’s why the leaf surfaces are greyed out for custom door leaf in previous versions (it is not locked in AC20, though).
If you set the surfaces of the base element to GENERIC before saving the custom part, the surface of the custom leaf becomes editable theoretically from within the placed door instances: the last material command before the custom component macro call takes effect. Unfortunately, this last material command refers to a frame surface in our current code structure (it is a known issue), but you can work around it inside the DoorLeafPanels macro by adding an extra line to the custom component script parts (around line 11195 in 3D script, material gs_leaf_mat fixes it). Note, this is a workaround with limitations, and works only with one surface (no inner-outer leaf surface option).
In case you have a locked surface parameter still, set the door panel to a style which unlocks the parameter, choose the surface, then change to custom door leaf.