I see what you mean. Yes, disengaging the intermediate gear will work just fine. That's what I have in mind too, except I would like to have a third postion to engage a secondary gear which reverses the feedscrew direction. Many 9x20 owners have done exactly this. Here is one example:

http://www.bedair.org/Tumble/Tumble.html

Also, I was a bit confused with your terminology. Typically, I see "crossfeed" refers to the motion of the cutting bit as it travels "across" or perpendicular to the axis of rotation. The motion of the bit traveling "along" or parallel the axis of rotation is usually called "Longitudinal" feed.