Don't do it.
You will break a number of third party addons which store settings in the Dex.ini. I know that anything I have written either under the Winthrop banner or under MBS banner uses the Dex.ini. This includes the Omni Tools, Omni Price, Named Printers, Advanced Security, Field Level Security and Australia's Business Activity Statement code.
The most common use for the Dex.ini is during the installation of the code, but I would still recommend against setting it for read-only.
I did write a solution to the problem with Winthrop's Omni Login product which will clear the SQLLastUser settingfor you. Omni Login will also remember the last company you logged into, unless you ask it not to.
David Musgrave
Senior Development Consultant
Asia Pacific Professional Services
Microsoft Business Solutions
mailto:dmusgrav@nospam-microsoft.com
Any views contained within are my personal views and
not necessarily Microsoft Business Solutions policy.