With DNGs in 21:9 the whole sensor data is there too but it is difficult to get at it in LR. I think you need a plug in. I just used a different program.
Hear you about SPP. It is kind of wacky (and slow) but it can do a nice job. It also offered some corrections that were needed with adapted lenses to deal with color casts. Loved the Rokinon 14mm on the sdQH but it needed SPP for corrections.