Thanks Aaron, that Lego coupler looks like it has possibilities.
Bigsmooth,
That's a possible solution. Thanks for the vendor link. The magnet(5053E) looks like a good size and thickness for a ceramic magnet. Too bad they don't provide the grade.
I went with the Brio hemi-spherical button in case I need to use a rare earth magnet. Don't want any pinched fingers. That nail head has rounded edges, so it would be ok.