Add a drop-down section under the Shaders list on the left which is a list of your TGCs, sorted by name, could mouse over for description or something, but when clicking a shader in your network, and than double-clicking a TGC, would add that TGC next in the chain of nodes, or if no shader select, just add the shader unconnected to the network. Maybe even using a button like the normal shade network for a option to add as child. The user can define a path in their config to a TGC folder, which would be recursively read for TGC files. Maybe a default location under Documents/Program install (Documents would be better for preserveration like User/Documents/Terragen/TGC Library)
TGCs are very important part of Terragen, both in work flow between projects, and sharing with the community, but there hasn't been much work done for TGCs since their inception. I've honestly wondered why no TGC library has been made on Planetside as an official resource for sharing and obtaining them outside the confusion of the forums. In the modern age, the average person doesn't know how to function forums anymore, and many are finding the systems cumbersome in general.
I feel this would save a lot of manual setups for TGCs within Terragen, and any new shaders you save (to disk within the path), would be handled.