Unfortunately this is not possible with what is available in Linux prep, Linux/UNIX guestOSes is 2nd to Windows in VMware's world and hopefully this will change sooner than later 
What you can do as a work around if you have or plan on installing VMware Tools when the guest is provisioned is to have a default password and then use something like VMware VIX API to change the password or upload a new password hash. It's not ideal but that's as automated as you can get today or by default provision your host with the proper password.
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William Lam
VMware vExpert 2009
VMware ESX/ESXi scripts and resources at:
VMware Code Central - Scripts/Sample code for Developers and Administrators
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