As this is definitely SIP-worthy, I created "SIP-22 NamedList Reduction" with an implementation plan that has already started (SimpleOrderedMap is now a Map). https://cwiki.apache.org/confluence/display/SOLR/SIP-22%3A+NamedList+Reduction
Please take a look everyone! At the moment, I leave open the question of API signatures switching to SimpleOrderedMap or further to Map. >
