Hello,
Basically you can use custom code in
TreeNode handler to change properties of document automatically after it's created/updated.
But I've tested creating document with diacritics characters in version 4.1 and diacritics characters were automatically replaced both in AliasPath and 'Live URL' in 'Properties'. Could you please let me know what version of Kentico CMS you use and where exactly the URL comes from, optionally provide us with some related screenshot(s)?
Best Regards,
Martin Dobsicek