What would happen if you made a slotted receptacle to make changing the angle as simple as lifting and dropping the seat in another slot? You already need to have a "stop" block installed inside the seat base to control the forward movement of the base of the seat. Why not multiple stops. I know it isn't a button but it wouldn't take much work to add a couple more.
Mine are at 12 degrees - but I like the sounds of that button for multiple reclining options. At one point, I tinkered with adding seat warmers in the passenger seat pad and backs... still working on that one...
GH
Dave Z has it right at about 15 degrees for the McKenzie double ender with transiom (see the guest seat schematic at the top of page 253 in the book). The only problem I would have with an adjustable back, as an oarsman, is the potential to bark my knuckles when having to heave on the oars. A shorter guy with less reach may be able to get by. Besides, if anyone gets too comforable up front they may forget what they are there for (to fish), or they may get too comfortable and decide to stay and overstay their welcome. Smile.
Roger, my apology, you have the 15 degree listed tilt listed on pg. 166 as well. I even read that section. I'm sometimes amazed that I have made it this far in life.
I have looked at 6-8 different sets of plans for drifters in the last few months, and must say yours are far and away the most professional. As I collect wood working tools and some skill I am looking forward to building a traditional wooden drift boat. Maby a double-ender. They have the fairest lines to me.