I'm just starting to experiment with using Magritte and Seaside in Squeak, and it's proving to be interesting if slightly hard going!
I think where I've got to with Magritte is that it's best thought of as a tool for building "edit" screens for objects (a task that it does very effectively under Seaside), but if you're trying to do anything else with your object, then it's back to plain old Seaside!
What I was trying to do was to put Magritte descriptions on an object subclassed from WAComponent, to allow me to use asComponent addValidatedForm to edit the object, but also use renderContentOn: to display the object. However, Magritte relies on a number of introduced messages on Object that WAComponent defines independently.
What I've had to do is introduce a 'wrapper' object to handle the view. This seems a bit cumbersome, but I've had a look at another Seaside/Matisse application — Conrad, a conference attendee management application developed by Stéphane Ducasse — and that appears to be the approach that he took. "Appears to be" as I'm having the usual problem of working out which way is up when looking at new code!