^self value asMIMEDocument
It allows me to use a block as a MIMEDocument so if my document renders itself immediately, I can delay it. I mistakeningly thought it was part of Seaside and not my own code! DOH! Anyway, the fix was easy. The documents are use for it render themselves on the fly and don't need to be wrapped in a block.
But, still this is an important lesson. AUTOMATE YOUR GUI TESTS! So, I'm going to do the two-pronged approach. Port the HttpUnitTest framework to Squeak and use it! Also, I need to download David Schaefer's excellent Seaside testing framework. Both would be a nice one-two punch. Well, I'm off to breakfast and a day of thrill rides.