@Cellarius is of course right. You need to use $prepared. You will also need to add your field to the array in 'class_userprofile.php". You should then be able to use it in your template as a conditional. Make sure you register array $prepared in your template or it will not work. Use hook 'member_build_blocks_start' -- which I already think you are using.
Around line #89 in 'class_userprofile.php'
Code:
var $auto_prepare = array(
..................
'show',
'field5', //Add your fieldxx here to the bottom of the array.
);
Sorry, I was too tired when I wrote this to see if there was another way to add 'fieldxx' to the 'prepared' array, but at least this will get you started.