How to customize discussion board in Sharepoint 2010 by editing thread.xsl

I don't think you need to modify the XSL, as these properties can be accessed by clicking on "Properties". However, it sounds like you will need to write an Event Receiver to create custom permissions on replies that grant the discussion creator access to it. The following can be a good place to start: http://ift.tt/1b2Fa9N

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Dimitri Ayrapetov (MCSE: SharePoint)