Choosing native screen resolution aspect ratio when screen sharing

I have two Macs both running Lion:

1. iMac (early 2009, iMac9,1 with NVIDIA GeForce GT 120)

2. MacBook Air (Early 2011, first of the new shape 11.6")

I often screen share using the Macbook Air as the client, and presenting the screen from my iMac onto it. When this happens the original iMac screen resolution is used and scaled down to fit into the lower resolution on the Macbook Air.

The iMac resolution native resolution is 1900x1200 (16:10), so when scaled down to Airs 1366x768 (16:9) it looks bad and doesn't fill the screen (even after using "full screen").

When I am in though, I can change the resolution, and have elected to put the display preferences menu bar item in to allow me to change quickly.

If I screen share using the normal physical display, I get the following choice of screen resolutions:

If I select to use the virtual display instead my choice of available screen resolutions changes to the following:

Oddly these are "extra" resolutions, but none of the available choices match the 16:9 aspect ratio of the Air, or equate to any of the supported 16:10 ratio resolutions of the Air, I only get options for the 4:3 aspect ratio.

Macbook Air Supported resolutions:

1366x768 (native), 1344x756 and 1280x720 pixels at 16:9 aspect ratio

1152x720 and 1024x640 pixels at 16:10 aspect ratio

1024x768 and 800x600 pixels at 4:3 aspect ratio

I want to avoid scaling wherever possible, and use one of the 16:9 ratios that my Air supports to get full screen non scaled viewing. I can get native vertical with horizontal borders at 1024x768, or I can get close to full screen after scaling with slim borders by using 1680x1050, but neither of these are ideal.

Is there any way to set my iMac to use these resolutions when I am connected via Screen Sharing? I know the root cause is a fundamental mismatch in aspect ratios, but surely the iMac can support a 16:9 external display, so why not for a virtual display?

After half a year and 2 bounties, no-one genuine solution, so the answer appears to be a simple "You can't do that".