I found a way to create a deployment that will override the default XSI_USERROOT. The trick is to add a set_spmhost.bat file to the deployment image.

The Softimage setenv.bat file is already set up to call set_spmhost.bat:

rem SPM_HOST
if EXIST "%XSI_BINDIR%\set_spmhost.bat" call "%XSI_BINDIR%\set_spmhost.bat"

so you just need to put something like this in a set_spmhost.bat file:

rem set_spmhost.bat
rem Override XSI_USERHOME from setenv.bat
set XSI_USERHOME=%XSI_USERROOT%\Autodesk\SI_2011_SAP

set_spmhost.bat is unused in Softimage 2011 and later.
It was used back in 7.5 for Linux customers who also wanted to run on Windows (back then, Linux licenses were still SPM).

In the next release, we’ve set up setenv.bat to call a SiteDeploy.bat file:

rem Verify the existence of the SiteDeploy.bat file
if exist "%XSI_BINDIR%\SiteDeploy.bat" call "%XSI_BINDIR%\SiteDeploy.bat"