There's absolutely no reason to break backwards compatibility when there are several perfectly good workarounds for this situation already.
One such example that comes to mind is simply changing the alias that is called when you double click on a name in the nicklist through Alt+O > Options > Mouse. Change it to something like my_query_alias_that_nobody_else_will_use and you can be sure it won't be overwritten
If the on open even was to be modified then perhaps there can be something similar to $menucontext so that the scripter knows how the window opened.
That way, all existing query blocker scripts that make use of on OPEN could keep their existing functionality by simply wrapping the code block in something like:
if ($opencontext == message) {
; code here
}
