You can do with SPTI everything you do with ASPI, it's just another interface.
You should :
1. Get the latest MMC commands specs from ftp://ftp.t10.org/t10/drafts/.
The command you're interested in is READ_DVD_STRUCTURE (+ and - will
require two different subfunctions).
2. Read Truman's sample SPTI C++ code in the Home made tools thread.
3. slightly change his code to call the function you want, parse the result