I posted some details on how I integrated Femtounit, my unit test framework for Interlisp, with the File Manager and the SEdit structure editor. It took much less work than expected thanks to a crucial insight buried in the documentation.
https://journal.paoloamoroso.com/integrating-femtounit-with-the-file-manager-and-sedit
Success!
I finally defined a new File Manager type for my Femtounit Interlisp unit test framework, then redefined DEFTEST in terms of it. Now the system notices and keeps track of new and modified tests.
Here's the File Manager type menu with the DEFTEST option highlighted under TESTS. The SEdit structure editor pops up this menu when editing a test.