C# - Calling Method On Master Page From WebMethod

, imagine a Master page that implements IFooMaster with a method

ShowFancyMessagePopupTheBusinessCantLiveWithout(string message);

I am in a page that inherits this master page. Upon a checkbox being unchecekd, I want to show a message to the user that if they save it, they can't re-check the checkbox without some admin action.

I've been given feedback that I can't just use an alert('message'); in javascript because they want the consistent look of these messages.

Next, I tried to make an ajax call via PageMethods (as that's what everything else in this codebase uses) to show a message. My problem lies in this method being static.

[WebMethod]
public static void ShowSuperImportantMessage()
{
if(!checkboxICareAbout.Checked)
((IFooMaster)Master).ShowFancyMessagePopupTheBusinessCantLiveWithout("If you uncheck that thing, you can't recheck it.");
}

Since ShowSuperImportantMessage() is static, I can't access Master from within. The method on the master page looks more or less like this:

public void ShowFancyMessagePopupTheBusinessCantLiveWithout(string message)
{
lblGenericMessage.Text = message;
btnGenericMessageOK.Focus();
upGenericMessage.Update();
mpeGenericMessage.Show();
}
mpeGenericMessage is an ajaxtoolkit:ModalPopupExtender.

upGenericMessage is an update panel.The other 2 are obvious.

Can I do some jQuery kung-fu to show that stuff? I tried, but the solution complained that the controls I tried to refer to by ClientID didn't resolve since they were on the Master page.

quick edit: Before anyone tells me the architecture is a problem, or I shouldn't have put such a thing on a master page, or w/e...

I know the situation is not ideal, but I this is inherited code, and I can't drop it all and rewrite half of their web stack.