When you load SeasideForWebToolkit from the public store you get a warning message telling you that you need at least 7.2. But you can disregard this warning and continue.
I you want to run under VisualWorks 7.1, you will be lacking a few features like uploading files into Seaside. You will not be able to run Seaside under VisualWorks 7.0.
-Information courtesy of Michel Bany