Hierarchy, Facets and Facet Plugins

Hey folks,

ElasticSearch looks really promising to me, and hits about 90% of my use
cases pretty directly. In terms of analyzing features, the main gaps I see
are around hierarchical facets
(https://github.com/elasticsearch/elasticsearch/issues/1076). I really
only need this around terms and statistical facets.

I saw some other threads around building facet plugins, looking at the API
it seems like it would be possible to write a custom plugin for this. It
would he hard to be as performant as the faceting in the core product
(mainly due to additional garbage that would need to be created around
building the hierarchy), but my guess is that I could hit my own
performance requirements.

Does this seem like a reasonable approach? Am I missing something? Has
anyone done this already?