From reading all this, you might get the idea that you can't run Selenium from SimpleTest. That's not the case. All you need is to call the PEAR package Testing_Selenium directly. It's slightly more typing than an API that is built into the test framework, but it won't kill you.
They list the steps you'll need to follow to get things working, but don't provide much more than that. There's a simple script included that you can test the installation with that makes a request to Google for the term "hello world design patterns" and asserts two different patterns to look for.