hi mike,
this is not a specific problem with DVD SP or any other authoring-tool – this is a BUG in the DVD spec according to presentation of your subtitles. did I say bug!? yes – you´ve heard correct, this is a bug!
if you would have it correct working you need to put a 16:9letterboxed and a 4:3 letterboxed version of your film on your DVD with the subtitles in the correct area.
and you need to read out the SPRM of your player, what setup is present in the system-parameters, and depending on this to jump to the different tracks.
now it´s up to you!
cheers
danny