Normally, I'm in favor of more script control, but I think it might be confusing to have a different abort key for each script. Windows has one abort key: ctrl+alt+delete. I've been using the <ALT> key as the script abort key, and it's pretty easy to get used to.
Pressing the <ALT> key to abort the script leaves you in <ALT> mode, which might be a little confusing, but it's consistent at least. You know you're using <ALT> to abort the script when you do that, and you expect to stay in <ALT> mode. Of course, you could abort the script AND exit <ALT> mode too if that seems better.
I do like having a modified <ALT> display line when a script is running.
While we're at it, how do you like the idea of also eliminating the shutter key as the script start key? I take a lot of unwanted pictures trying to start a script when I'm not in <ALT> mode. I'm thinking it would be good to make the first option in the script menu "Run Script". Pressing <set> once takes you to that menu option, and pressing it again starts the script. That is, you double click the <set> button to start a script. This also corresponds to the Windows double click of the mouse to run a program. You'd have to check the auto-select 1st selection for double click to work, but I don't think that's a problem. If you're changing the script abort key, this would be a good time to change the script start method if you want to.
None of this is a big deal to me, so whatever you want to do is fine.
[EDIT]alt_key_abort.patch : makes the <ALT> key the permanent and only script abort key.
I'd be OK with this, but in addition to the issues waterwingz brought up earlier, I think a broader consideration of what alt mode means is required (ref http://chdk.setepontos.com/index.php?topic=10589.0 )
There are actually 2 <ALT> modes. <ALT> with script not running, and <ALT> with script running. Pressing <ALT> currently always switches you to normal mode. I think that behavior shouldn't change.
In <ALT> script running mode, pressing <ALT> sends you to normal mode, with the script still running. Thus, we have a 4th state, normal mode with script running. In this state, the script sees keystrokes but they are also sent to the camera. Most scripts aren't aware of this, and there is no visible indication that the camera is in this mode.
The Lua functions, exit_alt() and enter_alt(), provide this same functionality but ensuring that the script is aware of it.
So I think the best solution is for the <ALT> key to always abort the script, but then switch you out of <ALT> mode. So when you're not in <ALT> mode, you know a script isn't running (unless the script is specifically designed to do that.).