Replying to my own post, I figured out the mean posterior probability calculation. My mind was stuck on calculating 'cumulative quantities. Iterating the following code, selecting row numbers that correspond to the given percentile, will match the output Miner gives for Mean Posterior Probability.
For the 10th Percentile (in the chart I linked to earlier -
http://econometricsense.blogspot.com/2011/02/sample-assessment-score-rankings-for.html )
%LET FROM = 10;
%LET TO = 20;
PROC MEANS DATA = VALIDATE;
VAR Predicted__TERM_GPA_Less_than_1_;
WHERE ROW_NUM > &FROM AND ROW_NUM LE &TO;
RUN;
I can't believe I bothered technical support over this. I just don't trust my results without solid documentation, or just checking the raw data myself.