[Users] Access to pseudo variables from within core

I have some questions about access to certain variables from within openser. The documentation states that I can access more of the pseudo variables like $fU and $fu from within more modules and the openser core. I am trying to use $hdr(From[0]) something that I can apparently still only do from within avpops or xlog. I need to have openser perform a certain action only if no SIP Display info exists if it does exist I do not want this action performed. For example....

if ("$hdr(From[0])") =~ "^<sip:.*") {.... do something } else { do something else... };

Or if there is SIP Display info do A else do B. Any help greatly appreciated.