Ok,

in the course of this overwhelming response I tried to find myself a
more or less flexible solution for an MDI app to block the master
datasets, callable within the OnStateChanged event of the detail
dataset:

procedure DisableMasters( Dataset : TIB_Dataset;
Form : TForm);
var
ds : TIB_DataSource;
cp : integer;
begin
ds := Dataset.MasterSource;
while Assigned( ds) do
begin
for cp := 0 to Form.ComponentCount - 1 do
begin
if Form.Components[ cp] is TIB_Grid
then
if (Form.Components[ cp] as TIB_Grid).DataSource = ds
then
(Form.Components[ cp] as TIB_Grid).Enabled :=
false; // true in EnableMasters()
{ the same for all other controls in use
(TIB_NavigationBar, TIB_UpdateBar, TIB_Edit ... }
end;
ds := ds.Dataset.MasterSource;
end;
end;

It actually works very well. But what to do with the ComboBox used to
set the OrderingItemNo? It's not data aware, so there's no DataSource
property :-(

Supposed I'm not the only one who uses a ComboBox to select orderings
that are too numerous and complex to be represented by the header
fields of an IB_Grid, would something like a 'TIB_OrderingCombo'
component be of common interest? Above all, there also would be no
more need to populate OnChange events and corresponding datasets'
OnOrderingChanged events for synchronization.

Regards

Christian