There is not any reference to custom aspect ratio on iMovie's manual, neither any setting in the project's or crop tool's properties.
If custom aspect ratios would supported from iMovie, I believe that Apple would make them easy to find..
After all, iMovie is a consumer product, and they have excluded such "advanced" features in order to get more customers to the Final Cut Suite.
The only solution to your problem, would be to crop your footage prior importing to iMovie.
You may use free apps such as MPEG Streamclip or VideoMonkey (which is the open-source version of VisualHub).
Although I haven't used any of them for cropping, they do support custom aspect ratios!
I hope this helps!