Thanks for the replies! ^
I went ahead and booked on expedia.com with no booking fees. I can lookup my booking on airberlin.com and ba.com. Manage my booking on ba.com says I'll get zero avios and tier points, even though my booking class E should earn 10 TPs and 25% avios per their chart... not that it matters much.
airberlin.com doesn't let me pick up a seat for free (though they seem happy to let me pay for it), for such a short flight not really a big deal, I'll check-in early and get what's available.