I have flown on a few of the Air Alliance 1900Ds, and they are a lot of fun to fly in, completely different experience than larger A/C (even DHC-8).
I can also assure everyone the 1900Ds which Air Alliance has DO
indeed have the aft lav option which when looking towards the rear of the cabin looks just like in this photo: View Large View Medium
Photo © William Mallinson
The lav door is split in the center and slides open to each side Star Trek style.
The first time I was aboard the 1900D, I checked out the 'facility', the lav uses the entire width of the cabin, and there is enough room for a 6-footer inside. It even has an electric blue-flush toilet, instead of a porta-potty.
If I recall correctly, the 1900D lav is much larger than the miniscule forward lav on the DHC-8.