My bus pirate arrived. To get familiar with it, I wired it up to an old Sparkfun LED Matrix I have.

I'm impressed. All I had to do was connect up the SPI lines, provide external power for the matrix and type things into the USB-serial interface.

The menu lets you type things like:

[ 0xFF:64 ]

Which does - Assert CS, send 0xFF 64 times, deassert CS. Almost too easy.

Here's a skull, in a bash one-liner:

I=7 && echo "[&:ff &:ff 2:64][ &:ff &:ff $I:24 $I 0 $I:3 0 $I:2 $I:3 0 $I:4 $I:8 0 $I 0 $I 0 $I 0 0 0 $I 0 $I 0 $I 0 0]" > /dev/tty.usbserial-A800ekql 

Of course, something had to go wrong. The LED matrix detaches from the SPI driver backpack. Sometime since I last used it (c. 4yrs ago) I must have taken it off then put it back on the wrong way around...