This is how I got SQL functions to work with the current Editor Library, it is HACKY but for what I need it works, might not be suitable for everybody and for every case, so tread carefully.

1) Edit Database\Query.php and change the _build_filed function to look like this:

    protected function _build_field( $addAlias=false )
    {
        $a = array();
        

        for ( $i=0 ; $i<count($this->_field) ; $i++ ) {
            $field = $this->_field[$i];
            
            // Keep the name when referring to a table
            if (strpos($field, ' fake_as ')) {
            
            $field=str_replace("fake_as","as",$field);
            $field=str_replace("|",",",$field);
            $split = explode(' as ', $field);
            
            $a[]=$this->_protect_identifiers( $field );
            $this->_field[$i]=trim(str_replace("`","",$split[1]));
            
            }
            else if ( $addAlias && strpos($field, ' as ') !== false && $field !== '*' ) {
                
                $split = explode(' as ', $field);
                $a[] = $this->_protect_identifiers( $split[0] ).' as '.
                    $this->_field_quote. $split[1] .$this->_field_quote;
            }
            else if ( $addAlias && strpos($field, ' as ') === false && $field !== '*' ) {
                
                $a[] = $this->_protect_identifiers( $field ).' as '.
                    $this->_field_quote. $field .$this->_field_quote;
            }
            
            else {
                $a[] = $this->_protect_identifiers( $field );
        
                
            }
        }

        return ' '.implode(', ', $a).' ';
    }

2) Edit the Field.php find the following line:

/** @var boolean */
    private $_xssFormat = true;

and insert the following text:

    /** @var saved_query **/
    private $_savedField = null;

In the same file replace the dbField function with the following code:

public function dbField ( $_=null )
    {
        if ( $_ === null ) {
             if (stripos ($this->_dbField, ' fake_as ') )
             {
                 $this->_savedField=$this->_dbField;
                 $this->_dbField=$this->_name;
             }
            return $this->_dbField;
        }

        if ( stripos( $_, ' as ' ) ) {
            $a = preg_split( '/ as /i', $_ );
            $this->_dbField = trim( $a[0] );
            $this->_name = trim( $a[1] );
        }
        else {
            $this->_dbField = $_;
        }

        return $this;
    }

Immediately after the dbField function insert one more function like this:

    public function dbSavedField ()
    {
        return $this->_savedField;
    }

3) Inside the Editor.php find the function _get ($id=null, $http=null) and change this bit:

    // Add all fields that we need to get from the database
        foreach ($this->_fields as $field) {
            if ( $field->apply('get') && $field->getValue() === null ) {
                $query->get( $field->dbField() );
            }
        }

to:

        // Add all fields that we need to get from the database
        foreach ($this->_fields as $field) {
            if ( $field->apply('get') && $field->getValue() === null ) {
                if ($field->dbSavedField()===null)
                    $query->get( $field->dbField() );
                else
                    $query->get( $field->dbSavedField());
                
            }
        }

Done with the editing, you can now simply specify your function inside the Field instance like this:

Field::inst( "DATE_FORMAT('2009-10-04 22:23:00'| '%W %M %Y') fake_as `clients.oneday`",'clients.oneday' )

Some notes, you will need to use | instead of , inside your field definition, also when naming your function use fake_as keyword instead of as and you must specify the alias one more time so Editor can name the fields correctly.

Enjoy.