The solution? Place a server control at the bottom of the page with CSS properties that prevent it from physically being viewed, don't affect scroll bars, and yet can still have focus shifted to it.
So begin by placing an ASP.NET button (or any other small server control) at the bottom of your markup, and add some inline styling to be most compact and streamlined if you wish:
Notice above the styling places it off the page, but the button will not create any lengthy scroll bars, etc. Lastly, add the code server side on the 'Finally' of the button click event to shift focus to the bottom of the page:
<!--Button will render, but not actually be visible to the client-->
<asp:Button ID="ui_btnPageBottom" runat="server" Text="Button" style="margin-left:-999px;" />
'Regardless of outcome, set focus to the bottom of the page