TO> Breakthrough: the Hierarchy classifier triggers the problem! Using
TO> the current custom macros with the Hierarchy classifier results in
TO> no buttons [...] Here are the title, green navbar, and
TO> "button" navbar HTML fragments:

TO> <td align=right><h2>de</h2></td>

TO> de [<-- between green spacer images]
[...]

Found the problem: unescaped quotes (") in the hierarchy file data.
This line was the culprit:

"Variations brillantes sur le Rondo favori de "Ludovic" de Herold "Je vends des scapulaires""20.3"Variations brillantes sur le Rondo favori de "Ludovic" de Hérold "Je vends des scapulaires""

Removing the line or backslashing the interior quotes fixes the
problem. Entirely my fault, though that is a surprising failure mode.

-Tod