Tomcat 7 does not support the RequestDumperValve that was available Tomcat 6 and earlier.
What is its recommended replacement in Tomcat 7?
And to answer my own question, more extensive Googling came up with this:
RequestDumperValve has been replaced by RequestDumperFilter, part of an effort to replace Valves with Filters to be more spec-compliant, and therefore more flexible. This is the class you want: org.apache.catalina.filters.RequestDumperFilter
Also see: http://tomcat.apache.org/tomcat-7.0-doc/config/filter.html#Request_Dumper_Filter
Note that you will configure this component in web.xml, now, and not in context.xml.