The DBI docs say:

Curently the DBI only defines two trace flags:

  ALL - turn on all DBI and driver flags (not recommended)
  SQL - trace SQL statements executed (not yet implemented)

So perhaps write to the dbi-users@perl.org and see what progress has been made or where you can jump in to patch a pure-SQL trace method for DBI. It's something generally useful enough that it seems like it should belong in DBI.