Is it possible to have the LogonStudio screen on logon and the native XP screen on logoff?
No.
By default when created logonstudio sets the margin for the messages to 0, you will need to edit the UIFILE of all the logons you use. Path for folder where logons are is C:\Program Files\WinCustomize\LogonStudio. Open logon folder, open the UIFILE in text editor, look for this section header near bottom of text /*CenterDivider*/, in that block find /*Logo*/
element id=atom(help) contentalign=wrapright width=0rp padding=rect(0rp,0rp,0rp,0rp)/
this line controls the messages, set width to at least 300, save changes, reapply logon.
element layout=filllayout() width=0rp , this line which is a couple below it controls Welcome, again set to at least 300.