The latest ARM version of uLisp, Version 4.1a, now adds support on the Teensy 4.0 and 4.1 for saving up to the full Lisp workspace with save-image and load-image.
This takes advantage of support for LittleFS recently added to the Teensyduino Arduino core by Paul Stoffregen.
To download the latest version see: Download uLisp.