To have Softimage start up with a certain layout, you don’t really have to do anything. When you exit Softimage, it writes the current layout to your preferences file, so that the next time you start Softimage it starts up with that same layout.
For example, if I change to the Tools Development Environment layout and exit Softimage, then my %XSI_USERHOME%\Data\Preferences\default.xsipref file will include this line:
xsiprivate.UI_LAYOUT_DEFAULT = Tools Development Environment
So when I start Softimage again, it will start up in the Tools Development Environment.
Here’s how to access that preference in scripting:
# Python
from win32com.client import Dispatch as disp
from win32com.client import constants as C
si = disp('XSI.Application' )
log = si.LogMessage
log( C.siUILayoutDefault )
log( si.GetUserPref( C.siUILayoutDefault ) )
log( si.Preferences.GetPreferenceValue( "xsiprivate.UI_LAYOUT_DEFAULT" ) )
# INFO : UI_LAYOUT_DEFAULT
# INFO : Compositing
# INFO : Compositing