I [think I] need to do a base 64 encoding in C# with a custom character set, but I can't figure out how.
Specifically, I'm trying use the Mixpanel HTTP spec - https://mixpanel.com/docs/api-documentation/http-specification-insert-data - from within a Unity3D project. Mixpanel says that it has a custom "flavor" of Base64 encoding, and indeed, when I encode the same JSON with a web service that lets me specify the character set (http://www.motobit.com/util/base64-decoder-encoder.asp), Mixpanel reports success.
(Thus, I know that the failure is in the encoding, not the data being encoded)