safe, universal, proceed addeventhandler javascript?

before i sum problem, i'd conditions clear. the web analytics association works consultant infinite sites, (other adding unparalleled book tag) have control over pages themselves.

our existent book installs handlers controlling "old" proceed (a imagination chronicle element.onclick = blah; also executes uncanny handler) totally unknowingly "new" (addeventlistener attachevent) handlers page. we'd repair the book means run some-more sites but requiring many tradition development.

the initial guess here have the possess book addeventlistener/attachevent, nonetheless presents problem: client's site sets handler controlling "old" way, clean out handler hallowed "new" way. discerning unwashed contrast shows happens both ie7 ff3, nonetheless i didn't exam whole operation browsers. there's also risk "new" proceed after page's eventuality handlers already set, erase handlers.

so doubt is: stable technique i supplement an eventuality handler javascript controlling addeventlistener/attachevent works regardless eventuality handlers page installed?

please remember: have proceed modifying site the book hallowed on. (i have stress since default answer questions always, "just rewrite page all same way.")