Can I set the sender/reply-to address in iOS mail for an ActiveSync account? I have a Google Apps account for mail.example.com, but actually use addresses @example.com (e.g. tony@example.com, whereas the account is tony@mail.example.com).  This isn't ideal, but I'm stuck with it for now.

In iOS mail I have this account setup as an Exchange account (i.e. not IMAP), which means that there isn't much in the way of settings.  Email I send from this account comes from tony@mail.example.com, which works, but I would prefer tony@example.com (this already works fine on the Desktop/browser).

Is it possible to change the sender/from/reply-to header in this situation?

(I wasn't sure whether this was an iOS mail issue and so should be asked here, or a Google apps issue and so should be asked on webapps.se, or if it's just not possible. Feel free to bounce me if appropriate!)
Unfortunately, no; it's not possible to specify a "Send From" address with an Exchange ActiveSync account. An Exchange Activesync account will always use the primary email address associated with it as the address the message is sent from.