I understand that a different combination of permissions will achieve basically the same goal. The purpose of this question was to get a definitive answer that could be mentioned in the extension creating thread and wiki.
Although tc-config sets up the /usr/local/tce.* dirs as 775 0:50, looking in /usr/bin/tce-setup, it appears that IF tce-setup is used (which it is during boot time loading of extensions if the bootcode base is not used) that ownership of /usr/local/tce.installed is given to $USER:staff before extensions are loaded. If tce-load is called separately (as it is during run time) the ownership will not be changed. This appears to be a possible inconsistency in the base.
Some clarification on the ownership and permissions of the files that could be placed in each of the /usr/local/tce.* dirs would be welcome as well.
Thanks for your input so far and I hope we can get this figured out soon.