TMySelectionEditor = class(TSelectionEditor)
public
  procedure RequiresUnits(Proc: TGetStrProc); override;
end;

procedure Register;

implementation

procedure TMySelectionEditor.RequiresUnits(Proc: TGetStrProc);
var
  comp: TMyComponent;
  I: Integer;
begin
  inherited RequiresUnits(Proc);
  Proc('ExtraUnit');
  // might be a better way of doing the code from here onwards?
  if (Designer=nil)or(Designer.Root=nil) then Exit;

  for I := 0 to Designer.Root.ComponentCount - 1 do
  begin
      if (Designer.Root.Components[i] is TMyComponent) then
      begin
        comp := TMyComponent(Designer.Root.Components[i]);
        if comp.SampleProperty = True then
            Proc('ExtraUnit2');
        Proc(comp.ObjProperty.UnitName);
      end;
  end;
end;

procedure Register;
begin
  RegisterSelectionEditor(TMyComponent, TMySelectionEditor);
end;