Thanks for your help.

"Write a '1'" is not possible because the microcontroller is not running any code during the production test.

Holding the device in reset should do the job but my supplier has had problems with that in the past. He used to put the controller in a so called "once mode" to do the job. Unfortunately this mode is not supported for the 89lv51.

To minimize risk, I changed my design using tri-state buffers for isolating the micro controller from the bus.