point to note: we did check the directory permissions on the share and we can access the share and setup files ok when breaking out of the setup.....
Not sure if you're still having a problem but I have noticed that BladeLogic puts in an entry called OemFilesPath in the unattend file.
In our testing we found this folder location it added didn't exist and caused the error you are getting.
We just replaced this using the custom unattend entries and added
to remove the line
Turns out when BladeLogic creates the date/time stamped folders under pxe\BLDRVDIR it does not set the permissions properly. So I'm just updating permissions on those folders whenever I create a new config.