The missing xml password has been found!

The post titled ISA2004 Installation Fails during SBS 2003 SP1 Install has been amended to include the solution to the missing password. Thanks once again to Jim Harrison for digging this information up, when PSS was unable to.

..and the answer is:

%programfiles%\microsoft windows small business server\support\sbsisa2k4setuplog.txt

..has the password embedded in it.