I found a roundabout way to accomplish this.
I put my timezone-specific timestamp in my pom's description tag like this:
<description>${maven.build.timestamp}</description>
<properties>
<maven.build.timestamp.format>E dd-MMM-yyyy HH:mm z</maven.build.timestamp.format>
</properties>
Now I can tell the pdf plugin to insert this into the coverdate. My pdf.xml has this:
<cover>
<coverTitle>My Cover Title</coverTitle>
<coverdate>${project.description}</coverdate>
...
</cover>
I now get the desired timestamp, in my local time, formatted as I want, on the PDF cover.
Admittedly, it's not so great to insert the timestamp into the description tag of the pom, but since I don't use the description tag for anything else, I can live with that.
Unfortunately, I can't access arbitrary ${project.X} properties in the pdf.xml... only a few properties like ${project.name} and ${project.description} seem to work.