Given a grasshopper network that extracts the line length of a bounding box, can this computed
value be queried from an external script in python? Basically I want to be able to get the value plugged in to the panel view from an external script.
You could use Hops - a component that allows you to communicate between the IronPython environment of Rhino (and Grasshopper) and CPython -, and either input the line that results from your Grasshopper definition, or even simply input the mesh and do everything in Python.