Mitch Wardrop,
Based on your post, you would like to customize the ClickOnce deployment by coding at the Publish property tab in order to add the authentication to the valid user account. I would like to provide the suggestions as follows:
1. In project property -> Publish tab, you can change the default configuration on the ClickOnce deployment. You can also make your own related user settings by changing the app.config XML nodes. Since you would like to insert your own code, please read the following whitepaper:
Administering ClickOnce Deployments
This whitepaper discusses all of these scenarios. It identifies the places you can insert your own code or tools into the process for control or monitoring purposes. Several alternative solutions will be discussed, allowing you to pick the approach that provides the best trade-off of complexity and control given your requirements. This whitepaper also identifies potential hazards of certain solutions that may seem like a good idea, but that can lead to fragility or unexpected results.
2. According to your question on authentication system, the most simple way is to create a Login form with the valid user accounts stored in MDB files. This is also easy for the deployment. The following article provide you the sample code on the Login form issue:
Provide a login form in VB .NET
Verify a username and password in a database in Visual Basic .NET
Please try to download the code and run the application. Hope that can help you.