Thanks for the suggestions, really helpful.

In the end, BA told me to search for and book separate legs. LON-DOH and DOH-DAR. For the latter, Qatar airways have not yet released their reward flights seats, and no idea when they would.

Having separate bookings means luggage won't be checked through LON-DAR, so we would need to clear customs, collect luggage, and check in again. Bit annoying, and possibly not possible if the layover isn't 4 hours plus.

Thanks again, really useful information from people here.