Change the custom field OptionSet value on reactivate case button from ribbon.

Hi,

I have a requirement that, if the case entity reactivate button on the ribbon is clicked then i need to change the custom fields optioset value.
adding web resource to existing system item on ribbon is what i thought would do but am not sure if thats the approach that needs to be taken or a plugin. 
Any guidance or code that will be able to accomplish this task will be appreciated.

Thanks
Gary