Have you ever tried using the RadAjaxManager? It was designed specifically for this purpose. While the RadAjaxPanel is a great solution for Ajax-ifying the content it contains, the RadAjaxManager allows you to specify which controls on the page update other controls on the page. So it would be very simple to set up a scenario where a button in a separate Panel updates a Grid in your RadMultiPage.
Check out this link
for an online demo using the RadAjaxManager and this link
for the help documentation.
I hope this helps. If you have any further questions, I'd be happy to answer them for you.