Let's see if I understand the situation.

You have various versions of a base prop. You have attachments that you want to smartparent (smart prop) to the various bases, but they must parent at a different relative position for each different base. Am I with you so far?

I'll assume the above is the case. I think this should work, so long as the bases have not been distributed already, and you are free to change them. For each base make a ghost prop (ghost = without any geometry), let's call the ghost prop "ParentA_1". You can use a primitive with an empty geometry reference as ParentA_1. Parent the ParentA_1 to the base, smart parent the attachments to ParentA_1.

When you parent something it is parented to the 'origin' of the parent item. You can position the ParentA_1 prop anywhere you like relitive to the base, the position of the ParentA_1 prop determines the position that the smart props will load relative to the base. Once the ParentA_1 is in the right position it should be Memorized, then locked so it does not get moved accidentally. The ParentA_1 prop is saved with each version of the base, in the same pp2 as the base, but in a different position relative to each different base.

When you save the base and ParentA_1 to the props pallet (using Select Subset) the base will not save as a smart prop, using "Select Subset" prevents that. You will need to edit the 'parent' line in the base prop to re-establish it as a smart prop. Replace;

parent UNIVERSE

with;

smartparent [name of parent]

I hope this helps, or did I completely misunderstand the question?