I just looked in my Knitting Without Tears by Elizabeth Zimmermann. She has what she calls an Afterthought Buttonhole in that book on page 40. She says in there that she had recently invented this buttonhole and that it "is chiefly aimed at experienced and courageous knitters". It involves snipping a stitch and unraveling a bit, binding off the resulting live stitches with the EZ sewn bind off. She says, "This buttonhole is not nearly as tricky as it sounds, but for goodness' sake make a few practice ones before you put the first one in an actual sweater." Good advice I'm sure. If you don't have this book and are interested in looking at what she says about the Afterthought Buttonhole in more detail, you can usually ask for it in most good library systems.
There may be other things you can do, but that came to mind.