So I skeined it up,
And prewetted it with just plain old water.
While the yarn was wetting itself (heh), Iboiled the coffee grounds for 15 minutes.
Then I strained out the grounds, washed the pot and put the brown liquid thus obtained back into the pot. Then I put the yarn in and put the whole thing back on the stove. 
I proceeded to boil it for 15 minutes and then I dumped it out into a colander.
Then I rinsed it in progressively colder water. The dye kept running so I decided to put a 'glug' (not very scientific, I know) into the rinse water. That seemed to do it and the dye stopped running. Or maybe it would have stopped anyway. I don't know, really.
The yarn did indeed take up dye from this experimental attempt. 
I was expecting it to be more chocolatey in colour. Instead, it's more of a sand colour. The descriptor that springs to mind is "the colour of soldiers' desert fatigues". That is, however, quite a mouthful. So "sand" it is.
Next time, if I do this again, I would put some vinegar in the prewetting soak and let it sit in the dye bath for longer than 15 minutes.
