I did a short where we shot with the 100a and did letterbox in camera, then imported using Vegas at the normal 0.9 aspect ratio, but when we put it on the timeline, we changed it to 1.2. This probably lost us a bit of resolution in the end but many people thought it was film thanks to some nice post color work on it. The stretching of 0.9 to 1.2 aspect ratio may do what you want it to, but the verticals will look a bit squished, so I would suggest framing as wide as you can.
I have the regular letterbox 4:3 version on youtube if you want to see it. Feel free to PM me and I'll give you the URL.
