You might try adding the following to the aircraft.cfg file:
[launch_assistance]
launch_bar_pivot = Xp, Yp, Zp
launch_bar_lug = Xl, Yl, Zl
where:
Xp, Yp, Zp are the respective X, Y, Z coordinates of the pivot (in feet)
Xl, Yl, Zl are the respective X, Y, Z coordinates of the launch bar lug (in feet)
Legacy aircraft probably don't have the animation of the launch bar built into the model, so don't expect to see the launch bar move when you hit shift-U . The numerical X, Y, and Z values are going to be unique to each aircraft. You can try guesstimating these values by looking at the nose gear contact geometry in the aircraft.cfg file. The Y values of both elements should be zero (0.0), since the nose gear is on the aircraft centerline.