I have no experience with Bang but how would you want the node to output the warnings? From here, you can see everything that can be extracted from a warning, including the description text and the element Id.
Going off the method @john_pierson gave and the linked doc collector from this thread Revit link elements, courtesy of @cgartland, this python code will give you a list of warnings with their respective elementid from all of the linked docs.
from Autodesk.Revit.DB import *
from RevitServices.Persistence import DocumentManager
current_doc = DocumentManager.Instance.CurrentDBDocument
links = FilteredElementCollector(current_doc).OfClass(RevitLinkInstance)
docs = [link.GetLinkDocument() for link in links]
all_warnings = 
for doc in docs:
warns = doc.GetWarnings()
warnings = 
for warn in warns:
elems = 
for elem in warn.GetFailingElements():
OUT = all_warnings
Let me know what you would want added to the list.
Edit: updated to return the actual element instead of elementid