I've always been under the impression, based on dire warnings from many sources, that even the tiniest speck of egg yolk will keep egg whites from beating to a stiff peak. "Oh, it just won't work if there's any fat in the eggs", we've all been told. This is beaten into the heads of every baker, and we all live in fear of getting yolks in the meringue.Well.....
After whipping it up, it looked perfectly fine, and unless I was hallucinating, it was holding a stiff peak.
Figuring that the horrible effect might happen after the butter was added, I went ahead and finished the icing. The butter went in fine, and I didn't see any difference between it and other batches I'd made ever while sweating about a speck of yolk.
Now I can't vouch for other things made with a meringue, and if you're making something that needs to hold the shape it might be different. But for IMBC, this is proof that if you get a tiny bit of egg yolk in the whites it isn't going to kill it. I don't recommend adding half a yolk, but a pea-sized bit per batch apparently makes no difference at all.