Thanks to some updates from Stuart, ArmedSlack now works on the Ultimate.
I have installed it to a USB key with only minor issues.
Minor gotcha - entering long config lines at the Openrd: prompt screws up unless you enable line wrap in minicom - this will apply to any install and is not specific to ArmedSlack.
Bugs: The XGIfb VGA framebuffer driver is missing - Stuart is aware, expect this to be fixed soon.
Both items now sorted - the line wrap issue is now documented in the install instructions and the missing framebuffer driver is now there and working.
So now you can have a fully working desktop with keyboard, mose and screen. Even X works albeit at 640x480 rather than 1024x768. This seems to be a driver issue but it might just be configuration.