This a sound approach, it ought to work.
Also, is it "block_expr" or "block_exp"?
The order in which your plugins are listed may be significant.

> I also considered adding the abstract text using the metadata.xml file,
> but didn't know what metadata name to store it under so that it will get
> searched with regular document text, rather than as a separate metadata
> field.

Well, you could store it in a field called (for example) "Description" and
then add an index to your collect.cfg file to search both [text] and
[Description] at the same time time.

Gordon