CRACKED IT ! My first successfully working Python API node.
I have modified the code and it working! @Einar_Raknes
I was walking home and had a light bulb-esque moment.
I transposed the graph, to produce a combined list with both the view and curve per item, then iterated through the originl script per item.
I was thinking this method could apply to any definition which will work only once, but for which you want to iterate multiple times.
Anyway enough chat, here's the code....
from Autodesk.Revit.DB import *
from RevitServices.Persistence import DocumentManager
from RevitServices.Transactions import TransactionManager
doc = DocumentManager.Instance.CurrentDBDocument
combinedlist = IN
outputlist = 
for i in combinedlist:
view = UnwrapElement(i)
curves = i
loop = CurveLoop()
for c in curves:
#Set cropregion in Transaction
view.CropBoxActive = True
OUT = outputlist