Maybe it should work; https://github.com/mitmproxy/mitmproxy/issues/319

doc-src/features/responsestreaming.html

@@ -40,6 +40,9 @@ Responses that should be tagged for streaming by setting their respective .strea
 
 $!example("examples/stream.py")!$
 
+In addition, if the .stream attribute is set to callable(), .stream will work as a hook in chunk data processing.
+
+$!example("examples/stream_modify.py")!$
 
 <h2>Implementation Details</h2>
 

examples/stream_modify.py

@@ -0,0 +1,11 @@
+def modify(chunks):
+    """
+    chunks is a generator that can be used to iterate over all chunks.
+    Each chunk is a (prefix, content, suffix) tuple. 
+    For example, in the case of chunked transfer encoding: ("3\r\n","foo","\r\n")
+    """
+    for prefix, content, suffix in chunks:
+        yield prefix, content.replace("foo","bar"), suffix
+
+def responseheaders(ctx, flow):
+    flow.response.stream = modify

libmproxy/protocol/http.py

@@ -1332,7 +1332,12 @@ class HTTPHandler(ProtocolHandler):
             # incrementally:
             h = flow.response._assemble_head(preserve_transfer_encoding=True)
             self.c.client_conn.send(h)
-            for chunk in http.read_http_body_chunked(self.c.server_conn.rfile,
+            for chunk in hasattr(flow.response.stream, "__call__") and \
+                            flow.response.stream(http.read_http_body_chunked(self.c.server_conn.rfile,
+                                                     flow.response.headers,
+                                                     self.c.config.body_size_limit, flow.request.method,
+                                                     flow.response.code, False, 4096)) or \
+                            http.read_http_body_chunked(self.c.server_conn.rfile,
                                                      flow.response.headers,
                                                      self.c.config.body_size_limit, flow.request.method,
                                                      flow.response.code, False, 4096):