Zhongren, I recall discussing this circumstance two years ago when on site. One of the conclusions we drew, was that the decision about which version of the item to use to fill demand would be an execution decision and not one that could be planned by EIS. That decision is made based on a variety of factors immediate to the execution that are not captured in the system.
As you suggest, in order to plan the group, you should aggregate the inventory, sales and forecasts into the lead item and calculate safety stock requirements for the aggregate as that item. These requirements can be met by any of the items in the group based on location, availability and so on but the volume of safety stock should still be optimized.
If you're aggregating the group in DP and SNP, that is ideal because you want to feed the aggregate numbers into EIS. EIS should be treating the group as one item, the lead item, and the decision about which item in the group to use will be made at the time of execution and order fill.
Note this is not a version limited issue because all version of EIS would handle this the same way.