The formula to find out a merit source is trivial, just give him/her enough time and will be giving out more merits than he/her could possibly have. There is no way to keep it a secret.
The secrecy of merit sources is assured by merit beggars being too stupid to think of that. Even after you just explained it, they still won't think of it, because most of them can barely understand English.
They wouldn't know it even if the list was pinned at the top of Meta. The average bitcointalker doesn't read anything nowadays - rules, pinned posts, or threads they're posting in. Beggars seem to send PMs to people they see meriting others with no particular concern whether they're merit sources or not.
I would argue that having the merit source list public would encourage accountability because:
The sad part is that a many of them are inactive