Problem with "Replace Scenario alternative for USMT Migration"

I've been trying to implement "The Deployment Guys", "Replace-Scenario-Alternative-for-usmt Migration"  This lets you capture data directly to a remote share on your newly imaged machine.

http://blogs.technet.com/b/deploymentguys/archive/2011/09/01/replace-scenario-alternative-for-usmt-migration.aspx

I've managed to get an MDT database installed with a couple of custom variables  configured.

I'm successfully pulling in the variables with the gather step. I've got my new build task sequence working with a step that creates a share adds permissions and test copies a log file:

net share USMT$=c:\USMT /grant:mydomain\%LegacyComputer%$,Change

cmd /c copy %_SMSTSLogPath%\smsts.log c:\USMT\%OSDComputerName%-%LegacyComputer%.log

But I can't get past the task sequence validate step, (run from the capture machine), that tests the remote share:

cmd /c copy %_SMSTSLogPath%\smsts.log %OSDStateStorePath%\2-USMT-Validation_successful_%_SMSTSMachineName%.log 

Access denied

It seems that I can't use a domain account from within Windows PE. (I'm running the capture in offline mode.)

Is there some permission I could add to my share that would allow access?

Thanks for any help or redirection.