code comments in a biblatex file

i don't know anything about biber, but here's what the bibtex manual says about comments:

p.13, item 7:

For Scribe compatibility, the database files allow an @COMMENT command; it's not really needed because BibTeX allows in the database files any comment that's not within an entry. If you want to comment out an entry, simply remove the '@' character preceding the entry type.

p.14, item 14:

LaTeX's comment character '%' is not a comment character in the database files.

the conclusion must be that there is no way to have an explicit comment within a bibtex entry, at least not the way that is expected in tex files.

however, if a field name is not recognized by the applicable .bst file, that field is ignored. so using a field name "mycomment" might be worth trying.

Biber ignores the characters of junk it complains about, so technically you can consider that still a comment. Biber will (contrary to BibTeX), however, ignore everything after a % without a warning.

So if you start a line (outside an entry) with %, that line is considered a comment by both BibTeX and Biber.

The BibTeX example file xampl.bib (described as "a database file with an example of every standard entry type") starts with

% Copyright (C) 1988, 2010 Oren Patashnik.

so that seems to be an "officially sanctioned" way to add a comment in .bib files.