Just a quick pointer to an answer on the Oracle Forum: http://forums.oracle.com/forums/thread.jspa?messageID=3912341#3912341
If you are merging into a data warehouse fact table (not so likely) or a large aggregation of a fact table (more likely) then you can improve efficiency by adding a transitive predicate into the USING clause to promote partition pruning on the target table.
Unless you are using Partition Change Tracking I think it is very unlikely that the MERGE inherent to a materialized view refresh is going to partition prune on the target table.
The Oracle Sponge 
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Just to let you know, even brief notes can be useful. This one didn’t solve my actual problem but it was a powerful nudge in the right direction. So thanks.