While playing around with answering an interesting question on dba.stackexchange.com I wrote a simple C extension to PostgreSQL that generates random
bytea values of a user-specified size. Fast.
In case anyone else is looking for a good way to dummy up random binary data in PostgreSQL, you can find the code in my scrapcode repository on GitHub
See the extension's README for details.
There's a pure SQL version fast enough to use for generating a few 100s of KB of data, too, or a couple of MB if you're patient.