You are not the first to run foul of the logon discovery process, it has caused me many headaches in the past...
What you need is a reskit util called setprfdc.exe (IE set preferred domain controller). Put a batch file, calling setprfdc and specifying the local DC, in the startup folder for all users. Note that from bootup the workstation will go through the discovery process and may still get the PDC rather than the local BDC. If that is the case, setprfdc will instruct the workstation to use the local BDC from that point onwards. Worst case scenario is that the first user after a reboot will have to login, have setprfdc run, then log out and back in again - by which time they will have been directed to the local BDC. Let me know if you have trouble tracking down a copy of this util - darthurs@hotmail.com
Hope it goes OK....