I am able to add the custom attribute to a toolbar button added in server side script. And, then, I can access the newly added attribute in order to immediately display the new value on a label. But, as you note, it does not get added to the HTML markup. See the attached image ("RadToolBarIssue.png") of the updated page in Firefox using Firebug.
sender, EventArgs e)
RadToolBarButton newButton =
//newButton.Attributes["data-attr"] = "CustomText";
lblInfo.Text = newButton.Attributes[
Does it have to be added in the rendered markup for your purposes? Or, can you access it server-side after adding it? I have not been able to locate the newly added attribute in the DOM of the rendered page. But, there should be a way to add it programmically. I'm investigating further.