This is what sucks about everything being scattered all over maemo.org. There's already an app, in extras-devel, that lets to make nothing pop-up at the camera lens opening. It's called cl-launcher (if you get it with apt-get), it shows up as Camera Lens Launcher in the Application Manager.

I am not a fan of sending people to extras devel as a whole, but I think for specific apps it's fine. If you're not already using devel, just add it briefly, don't update anything, download Camera Lens Launcher, and the remove the devel repository. The current cl-laucnher is pretty good and stable on my device. Some people have had a boot-loop from uninstalling it though, so it's may be a one-way install.

Then, you can set up a shell script, not my area of knowledge but I can learn it along the way and help you with it. And then find some way to run the script upon camera opening after setting the camera lens launcher to not launch the camera app upon camera opening.

One way I can think of this is setting up the script like a normal app, so that cl-launcher picks it up as an app, and then you can just set it as the preferred application to run upon lens opening in that same cl-launcher. Alternatively, you may be able to use dbus-scripts to respond to a dbus event from the lens opening. (For that matter, I would, with my minimal knowdge, look to dbus scripts to make the official-looking "Camera is disabled on this firmware; please consult your local Nokia carepoint for assistance" notice come up.)