How about playing with
preamble:
\usepackage{amssymb}
\usepackage{wasysym}
\newcommand{\cbox}{\ensuremath\square\kern-0.7em\raise.3ex\hbox{\checkmark}}text ctrl-m \cbox or making a math macro of that kind?I can see that there's also a kern command implemented in the math environment in LyX, but I don't know how to use it.
Yet another way would be to use http://www.fileformat.info/info/unicode/char/2611/ballot_box_with_check.svg
