Unable to set custom speed to motion path on Camera

This is a followup question to "Controlling speed along motion paths"
(Many thanks to Mark Spencer and specialcase for the their excellent insights getting me this far =)

After experimenting with motion paths and realizing they were indeed what I was looking for, I set up my project so that the camera would follow specific paths along 2d images. Everything worked perfectly until I tried to modify the speed along which the camera travels along those paths.

In the motion path, in the speed drop down menu, I can select Constant, Ease in etc.... all the way to Custom, but they work slightly differently than I expected: while before (ie. applying motion curves to objects as opposed to the camera) I was able to getting a diagonal line in the Keyframe Editor where I could modify keyframes using bezier curves to adjust the speed, now, choosing the various speed options appears to change the "Transform.Position.X" and "Transform.Position.Y" curves directly.

When I select "Custom", "Motion path" and Custom Speed" appear in the keyframe menu above the Camera drop down, but its curve appears only as a dotted, completely horizontal line in the graphing window. The "Transform.Position.X" and "Transform.Position.Y" curves also revert to horizontal dotted lines. If I try to add curves to the custom speed line, the X and Y curves remain horizontal and the camera does not move.

Is there some reason I cannot get custom speeds for cameras without losing my motion path data?

Is there some way to work around this?

Thanks again for all of your help,
Tim