select count(aux_42) from table for bloat analysis where idbpartition="(X.Y.Z)";

There is an effort to deliver VACUUM functionality to MCS.
This is one of the prerequisite to be able to count how many records has been deleted in a given partition.
As of 25.10 there is no way to query AUX column values directly b/c aux exists as a separate column in syscat calpontsys.systable table. It is scanned before any data column here and here.