You can test SNAPSHOT versions of the mojos tomcat6 and tomcat7 with the following configuration.
<repositories>
<repository>
<id>apache.snapshots</id>
<url>https://repository.apache.org/content/repositories/snapshots</url>
<releases>
<enabled>false</enabled>
</releases>
<snapshots>
<enabled>true</enabled>
</snapshots>
</repository>
</repositories>
<pluginRepositories>
<pluginRepository>
<id>apache.snapshots</id>
<name>Apache Snapshots</name>
<url>https://repository.apache.org/content/repositories/snapshots</url>
<releases>
<enabled>false</enabled>
</releases>
<snapshots>
<enabled>true</enabled>
</snapshots>
</pluginRepository>
</pluginRepositories>
<pluginManagement>
<plugins>
...
<plugin>
<groupId>org.apache.tomcat.maven</groupId>
<artifactId>tomcat6-maven-plugin</artifactId>
<version>2.3-SNAPSHOT</version>
<configuration>
<path>/</path>
</configuration>
</plugin>
<plugin>
<groupId>org.apache.tomcat.maven</groupId>
<artifactId>tomcat7-maven-plugin</artifactId>
<version>2.3-SNAPSHOT</version>
<configuration>
<path>/</path>
</configuration>
</plugin>
...
</plugins>
</pluginManagement>