In Maven's JarSignMojo, there is a check before it attempts to sign to verify
that the artifact's language is java:

http://maven.apache.org/plugins/maven-jar-plugin/xref/org/apache/maven/plugin/jar/JarSignMojo.html#204

From what I can determine, the signjar commands in
/hudsontrayapp/client/build.xml are never run.

The signjar executions in /hudsontrayapp/plugin/pom.xml are executed, but then
the code in the JarSignMojo will prevent the jars from actually being signed.
You see the evidence of this if you run mvn with the -X debug flag:

INFO] [jar:sign

]
[DEBUG] Not executing jar:sign as the project is not a Java module

[DEBUG] org.jvnet.hudson.plugins.hudsontrayapp:hudsontrayapp:hpi:0.7.3-SNAPSHOT
(selected for null)
[DEBUG] active project artifact:
artifact =
org.jvnet.hudson.plugins.hudsontrayapp:client:jar:0.7.3-SNAPSHOT:provided;
project: MavenProject:
org.jvnet.hudson.plugins.hudsontrayapp:client:0.7.3-SNAPSHOT @
C:\hudson\plugins\hudsontrayapp\client\pom.xml (selected for provided)