A workaround!

So there exists *one* way and one way only to safely set the Range header when using 64bit indices. This method was found with the help of NotZhila on #mono:

MethodInfo method = typeof(WebHeaderCollection).GetMethod
                        ("AddWithoutValidate", BindingFlags.Instance | BindingFlags.NonPublic);

HttpWebRequest request = (HttpWebRequest) WebRequest.Create ("http://www.example.com/file.exe");

long start = int32.MaxValue;
long end = int32.MaxValue +  100000;

string key = "Range";
string val = string.Format ("bytes={0}-{1}", start, end);

method.Invoke (request.Headers, new object[] { key, val });

You need to call the protected method of the WebHeaderCollection class so you can add the "Range" header without being forced to go through the broken HttpWebRequestAddRange method. I tried about a half dozen more legitimate methods, but the API is so locked down that it was impossible.

For example, HttpWebRequest.Headers is a get/set property, but if you set a new collection, it checks to make sure "Range" isn't there, and then just copies the keys from your new collection into it's internal one and ignores the collection you just gave it. What this means is that:

CustomCollection c = new CustomCollection ();
request.Headers = c;
Console.WriteLine (request.Headers == c);

prints false.

Ouch.

Anyway, the hack works. If you need 64bit indices when downloading files via HttpWebRequest, here's your guaranteed working hack, on both Mono and MS.NET.