Two comments:
1. I think there are two ground throws made by Caboose Industries, one for Code 80 and one for Code 55 track.
http://www.cabooseind.com/product-info They differ by whether they are "rigid" or include an internal spring. The idea is to use a spring of the proper tension to hold the points closed on the weaker Code 55 track without breaking it. (It is weaker in the way the rails are attached to the ties.)
2. If you want to insert your own spring in the rigid throw, then you will need to insert it between the Caboose Industries device and the turnout, where it will be visible. Considering that the Caboose industries ground throw mechanism is grossly out-of-(N)scale to begin with, maybe that isn't an issue for you. Or, you could mount the Caboose throw on the edge of the layout and connect it to the turnout throw bar with a long stiff wire, with a kink put into it to act as the spring. This is what is commonly done by people who use simple slide switches to both throw their turnouts and make the electrical connections to power frogs, signals, etc. I have seen pictures and descriptions of this in print, but a quick search did not give me a convenient link for you.