In reply to Peter Lancashire.

If you want only the missing value pattern, it's not too hard. List all classification variables on a CLASS statement. Also include the FCS statement. You will get a few warnings, but you can ignore them because you aren't doing imputation:

proc mi data=mydata nimpute=0;
var c1 c2 c3 x1 x2 x3;
class c1 c2 c3;
fcs;
run;