Thank you. In my usage scenario, I can assume that the users aren't malicious (but they could do some harm, as they aren't very tech-savvy). But the server side includes approach you suggested looks far better than my initial aproach. This way, I can just add content at the beginning of the file, and the users cannot break the layout, even if they submit an empty page.
If I'll have to code my own tool for this, I'll post the code in CUfP.
Again, thank you for your suggestions.