> We indexed each EAD guide into separate lucene documents for each EAD section, then collapsed them under the main EAD title in the search results,

Curious how you impelemented that: Did you use the Solr "field collapsing" patch that's not yet part of a standard distro?

> Matt and I were thinking it would be nice to allow blacklight to handle all of the display of the EAD too, 
> which is why we stored a lot of EAD markup in the solr document, and that can potentially have scalability
> problems, because lucene is not a database but we were treating it like one. This works, but it's a bit of a hack.

You can definitely have Blacklight handle the display while still keeping the EAD out of solr stored fields. There's no reason Blacklight can't fetch the EAD from some external store, keyed by Solr document ID (or by some other value in a solr document stored field).  That's my current thinking (informed by y'alls experience) of how I'm going to handle future large object stuff in BL, if/when I get around to developing it. 

Jonathan