One very simple solution might be to declare the columns as VARCHAR FOR BIT DATA. Preliminary tests show that in this case, I can store binary strings without explicitly specifying SQL_BINARY!

Update: After testing for a few weeks, I conclude that this solution works well for me. The only non-trivial part was to convert the VARCHAR datatype of the existing columns to VARCHAR FOR BIT DATA, as this requires dropping and re-creating the corresponding tables (as well as all related objects such as indexes and foreign keys). DB2 control center "db2cc" offers to do all that for you; however, be aware that it sometimes gets stuck in the middle of this conversion process---I highly recommend that you make a backup of your database before attempting the conversion.