No, there is no real way to configure the Tomcat version with the tomcat-maven-plugin. Actually, Tomcat 6 support is due to MOJO-1106 and was added in the version 1.0-alpha-2 of the plugin (see revision r7348), in previous versions and revisions, the plugin was using Tomcat 5.5.15. The version 1.0-alpha-1 is available in the repo so you could give it a try. But I didn't check and can't say if it could be impacted by any major issue.
If this option doesn't work and if you really want finer control, then it would be better to use Cargo as pointed out by romaintaz.