Q. How do I force the "Logon Using Dialup Networking" to be checked by default on the logon screen?

A. This can be accomplished with a registry change on each client machine.
Start the registry editor (regedit.exe)
Move to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
From the Edit menu select New - String Value (REG_SZ type)
Enter a name of RASForce
Double click the new value and set to 1
Close the Registry editor
Reboot the machine