Spamex forwards emails using a unique contact address, which allows you to reply through them and the sender sees only the original disposable that they used. Perhaps Fastmail will work on adding this in the future. It's a good start.
I do agree that a Fastmail address would be more acceptable to some people... although to me Spamex should not be classified with some of the others. They're also a paid service.
Of course FastMail also has the subdomain option if you need an email address in a hurry. (me@fastmail.fm can be used to generate
anything@me.fastmail.fm)