How should I create/configure a { C# | VB.NET } ASP.NET project in such a way to make it easier to develop and test a web-application targeted to basic WML browsers?
Use the .NET MobileControls; they output the correct form for the appropriate device (and as a result, are very limited in ability, but do what you need).
You can then also use the relevant emulators that probably come in some mobile SDKs to test on the different devices.