Thanks for the help here, once again. A quick bit of feedback: booking via the call center will incur quite a hefty surcharge. I managed to establish that the 'standard' fare on the web books into U-class, which is therefore eligible for segments (and a paltry amount of miles). However, I was prepared to do it just these once, as I really would like as many segments as possible early on next/this year (2014).
For DUS-PRG round trip, I bought for my companion a 'promo' fare at EUR99. For myself, I opted for the 'standard' fare at EUR220. Yes, I know it is a lot in comparison, but that is the sort of madness we get up to here (actually, I think it is perfectly reasonable for an airline ticket on this route). But I won't do this often and probably won't use OK all that much in the future.
I really wondered if there might be some fare classes between the 'promo' and 'standard' fares, but apparently none that earn miles!
Do I get the usual Gold benefits such as lounge access at DUS?