... The ap_hook_handler() call does not specify the handler key from the corresponding AddHandler configuration directive. As a consequence, the specified handler function must look at and accept or decline each request....
yep, see most handlers in the modules provided with Apache (mod_autoindex, mod_cgi, etc.)
Is there a rationale for this?
dunno
Am I missing a register_hook function that would allow me to set a key for a response handler?
no
I'm looking at trace statements and my handler must reject PNG files and so forth...
your handler needs to look at r->handler and decline stuff that you're not supposed to handle
