I don't see why it would not work.
The only thing you must make sure of is that you are running WinPE in X64 mode (Second step in the run script wizzard 64-bit not Auto-select) on a X64 capable machine.
I have posted our installation script we use for installing XP 64-bit below for your review.
Regards,
Dennis
REM Script for installing Windows XP + Aclient
REM
REM ITT Project Team
REM Current version: 1.0
REM Release: sept 2007
%systemdrive%\wndctl -hide
:Check Required Files
if not exist Y:\TOOLS\smartdrv.exe goto MissingFiles
:Preparing client environment
REM Replacetokens \\tudelft.net\clientrepository\deploy\templates\64bit\unattend.txt P:\Shares\Deploy$\temp\%ID%.txt
REM Replacetokens \\tudelft.net\clientrepository\deploy\templates\aclient.inp P:\Shares\Deploy$\temp\%ID%.inp
md C:\temp
F:\RDeploy\Windows\FIRM.exe COPY Y:\Tools OS:C:\temp\tools
md C:\Logfiles
F:\RDeploy\Windows\X64\FIRM.exe COPY W:\temp\%ID%.txt OS:C:\temp\unattend.txt
F:\RDeploy\Windows\X64\FIRM.exe COPY W:\temp\%ID%.inp OS:C:\temp\aclient.inp
F:\RDeploy\Windows\X64\FIRM.exe COPY F:\Agents\AClient\altiris-aclient-6.8.380.X64.exe OS:C:\temp\aclient.exe
del W:\temp\%ID%.txt
del W:\temp\%ID%.inp
REM bootwork unload
:Starting Windows XP unattended
Y:
CD \OS\XPsp264\AMD64
WINNT32.EXE /unattend:c:\temp\unattend.txt /s:Y:\OS\XPsp264\AMD64 /Syspart:C /Tempdrive:C /NOREBOOT
goto EoF
:MissingFiles
f:\wlogevent.exe -l:2 -ss:"Files missing on deployment server"
EXIT 1
:EoF