Save As for content retrieved via POST works in most circumstances.

content/browser/renderer_host/resource_dispatcher_host.cc

Index: content/browser/renderer_host/resource_dispatcher_host.cc
diff --git a/content/browser/renderer_host/resource_dispatcher_host.cc b/content/browser/renderer_host/resource_dispatcher_host.cc
index f39d36465d68a243d1284793f3302fe30ab78758..0c932cbc9a5c8a95e57115ea13b2edaba42c2457 100644
--- a/content/browser/renderer_host/resource_dispatcher_host.cc
+++ b/content/browser/renderer_host/resource_dispatcher_host.cc
@@ -893,7 +893,10 @@ net::Error ResourceDispatcherHost::BeginDownload(
   request->set_context(request_context);
   int extra_load_flags = net::LOAD_IS_DOWNLOAD;
   if (prefer_cache) {
-    extra_load_flags |= net::LOAD_PREFERRING_CACHE;
+    if (request->get_upload() != NULL)

[Randy Smith (Not in Mondays), 2012/02/02 19:46:10]

I think I'd like a comment here.  The form of the comment depends on whether we
think we want to long-term support POST in the non-cache case; if we do it
should be a TODO/bug reference, if not just an explanation as to why
conditionalizing ONLY_FROM_CACHE on get_upload().

[cbentzel, 2012/02/03 15:54:47]

I cover this with a comment in BeginDownload in download_manager_impl.cc. Do you
also want it here?

[Randy Smith (Not in Mondays), 2012/02/03 19:43:06]

I'm afraid so.  Code's better when it can be understood with minimal saved state
from other places, so I think we should have something (either independent
comment or pointer is ok).

[cbentzel, 2012/02/03 20:29:31]

Patch Set #9 adds a comment here.

+      extra_load_flags |= net::LOAD_ONLY_FROM_CACHE;
+    else
+      extra_load_flags |= net::LOAD_PREFERRING_CACHE;
   } else {
     extra_load_flags |= net::LOAD_DISABLE_CACHE;
   }