Forthrin, I could not find an elegant solution to this problem. But I did a little thinking, and a possible workaround would be outlined like this:
Make a directory called 'recipients', create dummy files named after the email addresses you desire to send to, use auto-complete inside this directory.
mail -s "stuff" 'firstname.lastname@example.org' < logfile.txt
Single quotes here to help avoid escaping the '\' that would appear.
I know it's not pretty, but you could doll this up by pulling from a text file of email addresses and cobbling an auto-complete together that way. I'm unsure how to go about that, though. Someone smarter than me could chime in.
Further, you can scour your previous history and maillog to look at the mails you've already sent and grab those addresses, then toss them into the recipients folder. Now you have a folder of recipients with which tab completion will work (satisfying your auto-complete request).
Not the best solution, but I don't see any feature in mailx for address books.