The two binaries for CentOS 5 seem to work on CentOS 6, they are binary compatible. More testing is needed, though.
When copying the files from the patch to replace the old ones, be careful and restore the proper ownership and permissions, otherwise check-quota won't be able to access the maildir under /var/qmail/mailnames/. You'd see a log entry like this:
check-quota filter[9413]: stat('/var/qmail/mailnames/example.com.au/adi/Maildir') failed. Permission denied: 13
When this happens, instead of "571 5.7.1 Mailbox full" Postfix returns "250 2.0.0 Ok" and accepts the message, then generates a "mailbox full" bounce back to the sender, which is the perfect backscatter example.
So perhaps Parallels should add some notes for this hotfix, otherwise your patched server might become a spam relaying machine.
* Later edit: I've just learned that Plesk is in breach of RFC 1893 -
http://www.faqs.org/rfcs/rfc1893.html - when deciding to return permanent failure for "Mailbox full" status instead of temporary failure. Currently the SMTP error is "571 5.7.1 Mailbox full" but it should probably be "450 4.2.2 Mailbox full"