I know this is a very debated topic, and usually when you are thinking about this as a solution you might want to rethink your UI logic.
I know I can pass validation using ClientScriptManager.RegisterForEventValidation. But, I'd really like to know. Is it possible to remove event validation for a single control? Is there a way work around for this?
I'm modifying a DropDownList, from the client side after it's rendered.
The short answer to your question is: No, there is no way to do this. You must disable EventValidation for the entire page.
There are various workarounds... If you really don't want to disable EventValidation, store the selected value in a hidden field and restore the state of the DropDownList (or maybe just clear selection?).
If you need all the values added client-side, you have to send those up using some other method anyway, because they will not be present in the server-side control. Those values are not posted!