Hello,
(Kostya, forgive me for asking again!)
I'm looking for a way to do this, but with Yahoo. I have Yahoo as my parent account and an identity (alias) of my name @mydomain.com as my default reply-to address. It is already set up with Yahoo so that when I mail out using chrome (or safari or whatever), the outgoing email is the @my domain address.
AM will allow a default send-from address, but it won't default to that when I reply to an email.
I know at one point (during many resets and new phones, etc), I figured out how to have AM show only my alias to me. In that case, it always replied from my @my domain address.
I'm pretty sure I figured out how to send from my alias one of the times, too.
Alas, I can't figure out how I did either of them.