Stein,

I dont think you will find much information on this online or in books unless you dive really deep into aero/hydro dynamics on a high level. Usually the area behind max thickness is for pressure recovery, so you dont want dramatic things to happen there. But adding some concavity will change the foils properties when you turn the rudders. I have not looked at it so I dont know, but there might be something to gain. Note that Tom Speer say that NACA0012 is a pretty good section for use in beachcat rudders, so the gains might be very small.
There are some posters on this forum who knows a lot about hydrodynamics (more than me at least), perhaps they can explain?
Magnus will probably be able to answer you in some years tough, when he finishes at NTNU <img src="http://www.catsailor.com/forums/images/graemlins/laugh.gif" alt="" />

If you want to know more about it, you will probably have to run some home-designed foils trough XFLR5 or similar software. Could be a fun project for some evenings, just start with e.g. NACA0010/NACA0012, add some concavity behind max. thickness and compare what happens when AoA changes to the default NACA0010/NACA0012 sections.
If you know what section daggerboards you have, it could be fun comparing they to what other profiles/AR ratios could do for you.