Tomcat: possible to exclude jars during app deployment?

By default, Tomcat prevents webapps from loading several .jars which are part of the Tomcat distribution (eg the servlet and JSP APIs) - is it possible to configure others to also be excluded?

If it can't be done with configuration alone, does Tomcat provide any extension points for resource validation?

I want to use shared logging libraries, and need to make sure that an errant app doesn't clobber the configuration.

Example

This is what I'm referring to (from Tomcat startup log):

Oct 1, 2011 5:53:40 AM org.apache.catalina.loader.WebappClassLoader validateJarFileINFO: validateJarFile(D:\tomcat\myapp\WEB-INF\lib\servlet-api.jar) - jar not loaded.See Servlet Spec 2.3, section 9.7.2.Offending class: javax/servlet/Servlet.class