As mentioned under the firmware tab I'm currently locked out of the BIOS. Luckily the way the S900 is configured lets me either boot via the network (PXE) or from a flash drive. In each case I've booted Tiny Core 4.7.6 which runs quite happily on the S900.
Revisiting in December 2019 I found that the S900 happily booted Tiny Core 10.1 from an 8GB SanDisk USB flash drive. It also booted the 64-bit version of Tiny Core which I used to confirm that it supported 8GB of RAM. (For some reason the BIOS doesn't report the size of RAM that it finds).
Till, who prompted the revisit, is running Ubuntu 18 on his S900.