With no direct resolution to this problem, I decided to instead add the dictionary terms to the main index.

I first added the dictionary terms to a new table in our database, this database being the same source of data for the main corpus of searchable material. (In this database, the added data is a tiny amount compared to the regular data.) I then indexed the dictionary terms with (a) fields that would cause them to be found as results and (b) a field to distinguish these dictionary term results from the regular results. Finally, I added logic to the calling code of the application which queries Solr, to be able to detect when a given result-set consists only of these dictionary terms. This lets me know when the user has supplied a correctly-spelled word, but which also does not match any of the regular materials.