BASE64 encoding is useful because it is a universal way to make all texts and binary files compatible with MHT format, which limits the symbols that can be used.
What kind of header customization do you need?
I am not sure if we are thinking the same thing when you say the Project Title. Just to make sure, what I mean is that this page`s title (on the browser) is called "MetaProducts Inquiry: MHT Export ..." When I save it as MHT, I would like it to be called that instead of what it currently is, which is "news."
I was just wondering about BASE64 as I noticed that when saving in MHT from Internet Explorer, they use something called "quoted-printable" which is composed of words and is readable, unlike BASE64.
The only other customization is the "From:" section in the header, which would just be a plus.