A common misconception is that anything with a central carving that is remotely egg-shaped was made by New Orleans furniture-maker/seller Prudent Mallard. While the egg device is characteristic of Mallard, it is not exclusive to Mallard. Below is a link to a photo of a bed attributed to Mallard showing his egg. I'm wondering if the 'split egg' on your mother-in-law's bed may be the imagined link to that famous maker (not that Mitchell & Rammelsberg is any slouch!).
GoAntiques Mallardhttp://www.goantiques.com/scripts/images,id,2221474.html#image2