add_filter( 'pb_whitelisted_domains', 'pb_add_iframe_srcs'); function pb_add_iframe_srcs ($whitelistedDomains){ array_push($whitelistedDomains,'vcu.kanopy.com'); return $whitelistedDomains; }
PressBooks has a filter that runs in prior to the kses filter and that was removing iframes that we wanted to remain. This bit of PHP in a plugin adds a single domain to the list. If you needed to add a bunch array_merge would let you do that easily.