Is it possible to use appcmd to change the value of allowDefinition? Specifically I\'m try to enable changes to the httpCompression module at the application level.
Try using %windir%\system32\inetsrv\appcmd unlock config -section:*sectionName*
. See http://blog.donnfelker.com/2007/03/26/iis-7-this-configuration-section-cannot-be-used-at-this-path/
I actually came across a need to do just that after posting this answer.
%systemroot%\System32\inetsrv\appcmd.exe unlock config /section:system.WebServer/[rest of the path to config section you need to edit]