For anybody else working in this area, I've put a fork of the Picoprobe firmware at
https://github.com/MarkMLl/picoprobe_led with substantially improved handling of both a single-colour LED and a WS2812 device (the latter on a WaveShare RP2040-Zero board).
I'd also note that I was talking to a Pico board on a Cytron "Maker Pi Pico". I eventually spent some time playing with some C code on this using cmake and VSCode, something that for some reason I had a lot of trouble with was getting the pinning right so that I could talk to the onboard SD-Card using SPI: for the record these should be MISO 12, CS 15, SCK 10, MOSI 11.
I don't know where we stand with embedding RP2040 PIO state machines in Pascal source or associated resources.
MarkMLl