Just did! Has only bootloaders from 2014, though, and firmware changes with the Pi itself mean it won't work on the latest Raspberry Pi models, nor will replacing the RISCOS.IMG from a current version with the Pico version.

When I tried RISC OS' own instructions to have the equivalent experience with their latest OS versions, they seemed to be missing a step, too, as I'd still boot to a desktop on a Pi 4: https://www.riscosopen.org/wiki/documentation/show/Raspberry...

Write the latest full RISC OS (through Raspberry Pi Imager or elsewhere). Edit CMDLINE.TXT per their instructions, and after it boots, on your keyboard, press F12 to open a command line at the bottom of the screen, type in the commands from their instructions, and then also type:

    Unplug Desktop

Then press Ctrl-Break to reboot. You don't need to copy files to other SD cards or anything. It will boot immediately into BBC BASIC in Mode 7 (40x25, Teletext pixels, 16 colors).

In swapping the SD card between a Pi 1/2 and a Pi 4, I was getting occasional misfires, though, and it wouldn't boot into BASIC, getting hung up on the wrong networking driver, or complaining about a missing desktop. Ctrl-Break to reboot should fix it.