A script like that won't work. The n900 compensates for different aspects of the audio system (speakers, headphones, earphones).

The reason it drops to 48% is because alsa mixer is set to place 48% of it's maximum volume 'value' to the earphones which in turn affects the headsets...You can't create a script to counteract this problem because just about anything on the n900 resets the value back to it's default (48%). These include screen dimming, blanking, and button pressing...

You would also notice that if you reflash the n900, the file which hold custom values for users (asound.state) is missing this is simply because the n900 does not use the file asound.state.

In short, Nokia has to fix the problem themselves because the area you would need to access in order to fix that (%) problem is closed source.