After a bit of a struggle, I've decided to give up on getting "edit record" working. The problem here is that in order to update a Solr entry, you need to rewrite all of the fields -- there's no true update, just a replace. Since VuFind can only easily write to Solr using the XML interface, and the MARC data in the "fullrecord" field of the index contains control characters that cannot be transported by XML, it appears to be impossible to save changes to most records. I've opened VUFIND-155 to track this -- if Solr opens up new HTTP-based protocols for adding records, this problem could become much easier to fix in the future. And if there's already some capability that I’m not aware of, please point me in the right direction!
Technical problems aside, I'm not sure that there's much value to this form of index editing anyway -- it might be useful for testing purposes, but in a production environment, direct manipulation of Solr entries seems more dangerous than helpful.
For the moment, I have replaced "Edit" with a "View" function -- at least this gives us a way to examine Solr records without having to go through the Solr interface; this might be useful in some scenarios.
I believe this puts an end to VUFIND-10, so I have closed it. I've opened a number of more granular tickets for various shortcomings of the admin interface, but I don't think any of these are important enough to worry about in the near term -- they're all currently assigned to the wish list.