Для тех, кто заинтересован, вот решение:
HtmlElement headElement = webBrowser1.Document.GetElementsByTagName("head")[0];
HtmlElement scriptElement = webBrowser1.Document.CreateElement("script");
IHTMLScriptElement domScriptElement = (IHTMLScriptElement)scriptElement.DomElement;
domScriptElement.text = "function applyChanges(){/*DO WHATEVER YOU WANT HERE*/}";
headElement.AppendChild(scriptElement);
// Call the nextline whenever you want to execute your code
webBrowser1.Document.InvokeScript("applyChanges");