One quick question: is this valid Asciidoc?  I guess so because
there
 are similar examples in documentation, but
    
http://www.methods.co.nz/asciidoc/userguide.html#X21
 tells:
   If the list contains any named or quoted attributes then all string
 attribute values must be quoted.
 Here, 'verse' is not quoted.  Can you please enlighten me? 
It's not valid AsciiDoc, but it works. So I'd say po4a should keep
supporting it.
Especially since the AsciiDoc user guide contains examples using this syntax!
And currently po4a doesn't support ["verse", more attributes here ...]
which it should. That's what I wanted to fix, but my first attempt didn't
work out, so I'll leave that for now.
/anders