so, lets create a new hook_signal($op) which fires at various points during a request. Initially, it replace hook_init() and hook_exit() with $op=precache, $op=postcache, and $op=exit. We shall also add $op=postbootstrap which is where the drupal_add_css calls will go.
This is an unneeded level of abstraction. We already have signals that fire at various points during a request. They are called hooks. They are faster and more descriptive than hook_signal($op).
hook_precache hook_postcache hook_exit
sure, thats a reasonable option ... i find it useful though to distinguish between hooks that care about your return value and hooks that don't. those that don't are candidates for grouping within a single hook_signal($op). the principle advantage is cutting down on the number of hooks ... not a big deal though.