I am plotting some matrices / arrays via MatrixPlot and ArrayPlot, and I would like to highlight several of the cells via outlining. Perhaps something like this (but less dramatic)

I've been playing around a little bit with settings for Mesh but it's a little unruly because it doesn't seem designed to allow specification at this level.
My evil desires would be to have a sort of HighlightFunction option that would make a decision to highlight the cell or allow a Highlight specification in ArrayPlot or MatrixPlot.
I know I can hand-roll this, of course, but was wondering if anyone had a solution 'out of the box' before I start.
The ToolTip inspired version linked below is a start at this but not a straight-forward solution for doing multiple, simultaneous highlights.