As far as that roll-up keyboard goes, I seem to remember seeing one for about $17 so they weren't all that expensive, HOWEVER, the main drawback to that type of keyboard is this: WHERE are you going to lay it so that it is supported enough? If you've got enough empty room to lay that thing flat with no interference (buttons lying underneath, joystick curling up an edge, etc.) then you'll have enough room to put a drawer in the backside of your CP for a mini keyboard which should be easier to use. That's essentially the long-winded version of "that keyboard seems wack, go with another idea".
That crane thingy is exactly what I was thinking of! I think you'll have to see one in person to see what directions you'll be able to move it in after it's mounted. That'll also help you figger out if you can move it down and out of site when not in use.
What I'm picturing on your CP is this: 24" wide leaves plenty of room in the back half to install two drawer slides w/a shelf inside, and mount a keyboard to it. My keyboard is 7 inches wide, and it's pretty standard, so at most, you've got a keyboard that's 1/3 the width of your CP, so plenty of room to store it inside your CP...also, consider a mini keyboard...even less space.
The thing I have in mind is here
http://www.rockler.com/ecom7/product_details.cfm?&offerings_id=55and instead of using it how they have it pictured, imagine reversing it so it pulled out and angled up, kinda like at a 45 degree angle to you....are you thoroughly cornfused yet?
Lastly, wireless keyboard would be the way I would go in case none of these appeal to you. I've been using a wireless keyboard for about 6 years now, and with a set of rechargable batteries, the thing is golden! See if newegg or someone has an older Logitech version on sale. DO NOT GET THE INFRARED TYPE, ONLY GET IT IF IT SAYS IT'S THE RF TYPE, TRUST ME! After a week with an infrared one, you'll need to replace your screen after you throw those POS infrared types through it!