We can't be of much help with the history feature improvements, as it's part of the standard ScriptManager functionality. You can edit MicrosoftAjax.js, save it as an external file and reference it through RadScriptManager:
This file is different for ASP.NET 2.0, 3.5 and 4.0. The source code (debug and minified versions) are available from Microsoft:
The 2.0 version is distributes as part of Microsoft AJAX 1.0.
I hope this helps.
the Telerik team
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