What you can do is to use a long piece of L shaped wire that locks into the rudder, preffibley under the control horn and extends through the fuselage. Then put a double horn on it and two springs to a horn on the tail wheel. Another way would be to run a second pushrod from the rudder servo to a a tail wheel, or even use a mini Servo on a Y harness near the tail wheel and couple it to the tail wheel in a pull-pull arangement. That would require a tail wheel a bit different than the Sullivan. something like this
http://www3.towerhobbies.com/cgi-bin...&I=LXMRK6&P=ML

I think the last suggestion is the best, but as you can see, there are a number of ways to approach this and I'm sure more will pop up in this thread.

Don