The MathJax TeX fonts don't include the necessary glyphs to make this possible. But the STIX fonts do, so if you are willing to use those fonts, you could manage it. Since the STIX fonts are only available in the HTML-CSS and SVG output, you could not use the CommonHTML output renderer for this.
Here is an example:
<!DOCTYPE html>
<html>
<head>
<title>Implement \widecheck macro</title>
<script type="text/x-mathjax-config">
MathJax.Hub.Config({
"HTML-CSS": {fonts: ["STIX"]},
SVG: {font: "STIX-Web"}
});
MathJax.Hub.Register.StartupHook("TeX Jax Ready", function () {
var TEX = MathJax.InputJax.TeX;
TEX.Definitions.macros.widecheck = ['Accent', "02C7",1];
});
</script>
</head>
<body>
<div>
$$\widecheck{xyz}$$
</div>
</body>
</html>
Perhaps that does what you need.
Davide