I think the best way to achieve this would be to write an application that ran continuously on the Slice hardware. To write to the LEDs you just use the open() function to open /dev/ws2812 when you want to write to the LEDs
Then you use the write() function to write 100bytes to the device and then close the device...
The 100 bytes are 4 bytes for each LED, which is a RGB32 value (i.e. one byte of red one byte of green one byte of blue and then a padding byte)
If you just write 4 bytes then it'll just change the first LED... Have a go..