Edit: It works on boards using espressif modules but I cannot seem to get this to work for a board directly using an S3R8
A solution!
If I configure the psram as suggested in: https://github.com/esp-rs/esp-hal/issues/3956 and https://docs.espressif.com/projects/esp-idf/en/stable/esp32s3/api-guides ...

I have enabled psram in Cargo.toml:
esp-hal = { version = "1.0.0", features = ["defmt", "esp32s3", "unstable", "psram"]

Dutifully following the instructions at https://docs.espressif.com/projects/rust/esp-hal/1.0.0/esp32s3/esp_hal/psram/index.html, I have put:
esp_alloc::psram_allocator ...