I was struggling with this and couldn't figure out why it wouldn't work. Finally I got it. For others who might have the same issue:
The above advice doesn't say it explicitly but when setting up the new account, you must select account type as "Other", not one of the alternatives such as "Gmail" (as I was incorrectly doing).
Then on the first screen, enter the address and password of the account you want as the "reply to:" address. On the next screen, enter the account data for the incoming and outgoing servers. For the incoming server, use the account data of the account you want to receive on the iPad (which in my case was indeed a Gmail account, using mail.gmail.com as the server). For the outgoing server, you must use the actual SMTP server of the account address that you want to have as the "reply to:" address (in my case, smtp.gmail.com).
For example, my incoming account is a gmail account using the "mail.gmail.com" server with the appropriate account id and password. My outgoing account uses my company's smtp server ("smtp.mycompany.com") with my corporate account id and password.
It works fine. The only thing is to realize is that you must have authorized access from the outgoing server for it to work.There's no way to just type in any old "reply-to" address as you can with most email clients.
PS. I don't understand why those who don't have an answer to an OP's original question don't just keep quiet instead of filling up the thread with snotty "you can't do that and why would anyone ever want to do that" posts. Really wastes a lot of other people's time as they try to read through a thread searching for an answer. Sigh. Just how people are, I guess.