type
TFormExt = class(TForm)
private
***
protected
{ Protected declarations }
public
***
procedure Doanything( var Value: Variant ) virtual; abstract;
***
published
{ Published declarations }
end;

***
procedure Doanything1( var Value: Variant );
begin
***
end;

procedure Doanything2( var Value: Variant );
begin
***
end;

***
procedure TForm.Btn1Click( ... )
var
Value: Variant
begin
(fMain.ActiveMDIChild as TFormExt).Doanything( Value ) := Doanything1( Value );
end;

procedure TForm.Btn1Click( ... )
var
Value: Variant
begin
(fMain.ActiveMDIChild as TFormExt).Doanything( Value ) := Doanything2( Value );
end;
***