In testing, I tried dropping the user control into the content area of the
content page.  Now the compiler knows that my content page knows about
MessageBlock controls.  Next I deleted the user control from the content
area.  It still compiled.  That left me with  <@ Register Src="whatever" %>.  
The <%@ Reference %> from your suggestion is even better and works.  Thanks.

I do have a question about your first suggestion, though.  I could probably
work this out but a quick tip might save me a lot of time.  

When you say to put the control into a class in app_code, do you mean to
create the item as a web custom control rather than a web user control, and
create all the rendering code myself?  Or do you mean to create a class that
has the user control as a public property or exposes the user control through
encapsulation or delegation?

Thanks,

Dale