JF, could another possible explanation be the trick of using a Gurney flap to aerodynamically increase the apparent size of the rudder, as on the MRA 4?
An example of aero trickery from
http://www.dept.aoe.vt.edu/~mason/Ma...A2007-4175.pdf
Trailing edge T-strips have been used to improve the performance of aircraft vertical tails. An example is the use of T-strips on the vertical tail/rudder of the Sino Swearingen SJ30-2 business jet to increase dutch roll damping. Flight tests showed that a 1% chord T-strip on the lower third of the rudder increased dutch roll damping for the flaps up, 180 KIAS flight condition from essentially zero to 0.12, more than double the FAR 23 minimum.
Edited to add - Testing the Honda Jet
http://www.icas.org/ICAS_ARCHIVE/ICA...PAPERS/437.PDF
During the initial flight testing, the airplane exhibited a small amplitude rudder oscillation in the mid-speed range. To eliminate this, various sizes of T-strips were added to the trailing edge of the rudder. An example of the effect of the partial span T-strip on the rudder oscillation is shown in Figure 22. A T-strip eliminates the oscillation (Fig. 23).
What is the real reason for the use of dome headed rivets on the rear fuselage skinning
Perhaps the boundary layer is so thick at that point that the effort to countersink is not worth the meagre return.