This is now possible out of the box in the floccus settings.

Using Perl

$ perl -CSD -pe 's{(://.+\))}{$a=$1;$a=~s/\w/X/g;$a}ge; s{(\[.+\])}{$a=$1;$a=~s/\w/D/g;$a}ge'
  floccus-4.6.4-2021-07-16.log > floccus-4.6.4-2021-07-16-anonymized.log;