This is NOT a crippled version

It’s actually very easy to change the aspect ratio in this Mac version, although it’s not readily apparent. According to the short Help section on their support site (hightly recommended reading for useful tips):

"How do I create a customized layout size?
Go to the Finish screen and scroll to the export size options. Enter the largest dimension size (for example: if your desired output was 200x300, you would enter 300 in the right box) in the proper box (left is width and right is height). Then, use the Aspect Ratio slider to achieve the desired dimensions. Moving it to the left will create tall and skinny Diptics, and moving it to the right will create short and wide (landscape) Diptics. You can also manually type the desired ratio in the box to the right of the Aspect Ratio slider."

I just tried it to make a collage for a HDTV presentation, setting it to a 16:9 aspect ratio, with a size of 1920x1080 px, and it worked like a charm.

ticocats about Diptic, v2.0.4