<p>What resources or methods would OpenSCAD users suggest to piece together disparate STL files?</p>

<p>I'd like to take an existing STL model-library of STL parts (head, torso, arms, legs) and make it a Thingiverse OpenSTL maker, similar to the castle generator and/or the puzzle generators available.</p>

<p>This way, users can generate a custom model using the designated parameters and download the model for printing.</p>

Cura prior to version 2.5 does not take G-code as an input. I'm not sure what you mean by "print only using Cura." Don't you have a SD slot on your control board? For that matter, why can't you drive the USB port from Slic3r?
Remember: G-code is the equivalent of "compiled code," the raw commands which drive the printer, while STL or OBJ, etc., are the "source code," which you edit to get the shape you want.
I've looked at a few apps which will render a 3D image on your computer from G-code, but I don't know off-hand of one which will "decompile" into a STL or mesh file.