I have no idea what experiment you just did, or what the software you are using will do.
I did an experiment that shows that the plugboards do what they are supposed to do. I did a save-to-disk of an epub book in my library. According to my debug log, my plugboard contains:
Code:
Save-to-disk using plugboard: epub [(u'{title_sort}bar', 'title_sort'), (u'This is a template', 'title')]
which is exactly what I put in the plugboard <any format> <save to disk>.
The OPF in the epub (not metadata.opf) in my library contains:
Code:
<dc:title>1633</dc:title>
and does not contain a title_sort attribute. The OPF in the epub copy saved to disk (again, not metadata.opf) contains:
Code:
<dc:title>This is a template</dc:title>
<meta name="calibre:title_sort" content="1633bar"/>
Clearly the plugboard changed the title and the title sort.
What are you doing different from the above?