Pete, on LEU Treehugger, I have springs that connect to the lower Sta 1 horizontal tubing and about the mid-point of the vertical part of the rudder pedal. The spring has a slight stretch when pedal is full forward. The spring has to be selected for the right length, diameter and wire diameter so it provides reasonable tension and also can be stretched to its fullest extension when the rudder pedal is full back, and still not be permanently deformed. I tried several to get the right ones.
In my thinking, the rudder cables should always be under some tension, as for example, you push right rudder, and pull back your foot on the left pedal, the left cable will have some tension due to the spring. On that note, be sure that any pulleys that the cables go through have keepers that will not let the cable get out of the pulley groove.
I have not tried the spring mounting like you show, but from your description, it works like I describe above and should be OK.