Trabaja alrededor de esa funcion, dale debug a la pagina, imprime por ahi el contenido del stringbuilder, porque deberia funcionar. El Page.RegisterStartupScript, segun el msdn hace lo siguiente:
Código:
Allows ASP.NET server controls to emit client-side script blocks in the Page.
[Visual Basic]
Overridable Public Sub RegisterStartupScript( _
ByVal key As String, _
ByVal script As String _
)
Similar to the RegisterClientScriptBlock method, this method emits the script just before the closing tag of the Page object's <form runat= server> element. The script block is emitted as the object that renders the page is defined, so you must include both tags of the <script> element.
By identifying the script with the key, multiple server control instances can request the script block without it being emitted to the output stream twice.
Any script blocks with the same key parameter values are considered duplicates.
Note Remember to include HTML comment tags around your script so that it will not be rendered if a requesting browser does not support scripts.
Consulta un poco de la ayuda (no se si delphi traiga documentacion del framework), pero tengo ese codigo trabajando en muchas paginas, registrando scripts que presentan alertas, abren ventanas, etc.