There is an available filter rul_replace_variable
for adding your own custom placeholders.
Say you want to add a {{month}} placeholder that is a numeric representation of the current month (with leading zeros), you can use the code below.
function customRULVariableMonth( $value, $variable, $user )
{
if( 'month' == $variable )
{
return date( 'm' );
}
else
{
return $value;
}
}
add_filter( 'rul_replace_variable', 'customRULVariableMonth', 10, 3 );