Thinking some more, this might not be the best approach. It is very seeky on the read path.  Given how small the index pages will be, we could also consolidate them all for a row group to be contiguous on disk. The row group metadata would have the offset/len for all the index pages and the offsets in the entries would be relative to the index data start.

The reader would then just read this whole thing and and keep the indices of all the columns in RAM. This would still allow very seeky read patterns if the query was projecting very few columns.