Many historic Greenland kayaks had a skeg that was tied on and used only when the conditions warranted it. You can still do this if you want- here's a modern version of the strap-on type skeg:

http://www.rollordrown.com/kayak/skeg.html

But you could also do it a little bit differently and add some sort of receiver strip on your hull, and make an add-on skeg like these:

http://capefalconkayak.com/The%20modern%20skin%20on%20frame%20kayak.html

http://kajakkspesialisten.no/e_annet_tips.php

I think either solution would be better than a drop-down rudder-type skeg, and you still wouldn't have to cut a hole in your deck, only some screws, or even adhesive sealant if you choose. If you've got a solid roll, if you're on the water and decide you need it you can even put it bottom-side up and let a friend install the add-ons and roll back up.

m