It should work from the service order activity as well
As stated by your customer and given the scenario you described earlier the system should also give the message tssocs2100 in case the service order activity (with the field 'Change Method' set to "Install") is set to "Costed" and no installation has been performed yet.
In fact from the service order activity a new message should come up after you have answered 'Yes' to message tssocs2100.
This next message is tssocs2075 (All lines for the order are costed. Change order status to Costed?). If you click "Yes" here as well the status of the service order itself is changed automatically.
Of course you can also change the status of the service order to "Costed" after message tssocs2100 at the service order level.
I tested your scenario on our machine and the system does exactly the way I just described it.
Since you are not getting the message tssocs2100 at the service order activity level I advise you to find out whether you are using the latest object of tssoc2110. Make sure you have installed the latest patch from BAAN Support for session tssoc2110s000.
Let us know if this worked for you and if you have any other questions please put them on BaanBoard.