Looks like there is no easy solution. Here is how I manage for now. It's neither automatic nor perfect and requires a little discipline but gets you a good part of the way there.

For cropped images, I used hierarchical keywords since Smart Collections are not flexible enough yet to distinguish between exact aspect ratios. Like this:

This is the part that requires discipline since, every time an image is cropped to one of these unusual-but-even aspect ratios, the right keyword must be applied. For example, images with 2:1 aspect ratio get the 2x1 keyword.

This obviously leaves out images with odd aspect ratios. Lucky for me, I only crop 1.2% of images.

For standard aspect ratios, I created an Aspect Ratio Collection Set and Smart Collections for each aspect ratio. Like this:

Above 3x2 is a Smart Collection which selects on:

• Cropped is False
• keywords does not contain AspectRatio
• Camera contains DSLR K-5 K-7 K20D K10D EOS REBEL S5Pro E-5 E-620 SLT D3 D300 ...

Where .... is actually a long list of Nikon DSLR, since like Pentax, there is no identifying keyword for all of them. In the case of the 4x3 collection, that would be:

• Cropped is False
• keywords does not contain AspectRatio
• Camera contains PowerShot Coolpix Finepix DiMAGE DMC DSC E-P1 E-PL2 Photosmart Optio Tough Stylus X70 XZ-1 Exilim EasyShare
• camera does not contain S5Pro

It is a little easier since fixed-lens cameras are grouped into families by nearly everyone, for some reason you will notice the Pentax X70 does not say 'Optio'. Also note the Fuji S5Pro is part of the Finepix line, so it must be excluded explicitly.

Excluding the AspectRatio keyword hierarchy from these collections lets me label images that come out of cameras with a strange aspect ratio. A number of compact cameras for example support Sweep Panorama or Automatic Panorama features that produce images that do not correspond to the sensor's native aspect ratio. The same is required for multi-ratio cameras such as many Panasonic Lumix models. When such an image gets imported, simply labeling it with the correct AspectRatio keyword such as 7x1, excludes it from incorrectly appearing the the Aspect Ratio Smart Collection.