After some looking around and experimentation, I have found the answer to running the GroupWise 8 WebAccess & GroupWise 8 Publisher on the same Tomcat 5 instance that iManager 2.7 uses.
Although the Tomcat5.rpm from our distribution is necessary to install the WebAccess for GroupWise 8, It only has to be there when you start the WebAccess Configuration. After starting the WebAccess configuration, simply point it to your existing iManager 2.7 'webapps' directory, complete the configuration process, and then when you are done, make the following changes to the '/etc/apache2/conf.d/gw.conf' file. (What it puts in here is for the standard distro install of Tomcat 5)
Change the first two lines where is says:
ProxyPass /gw/webacc ajp://localhost:8009/gw/webacc
ProxyPass /gw/webpub ajp://localhost:8009/gw/webpub
so that it now says:
ProxyPass /gw/webacc ajp://localhost:9009/gw/webacc
ProxyPass /gw/webpub ajp://localhost:9009/gw/webpub
Notice the port number change.
Stop and start stop your Tomcat5 and Apache2 daemons now and you're all set.