The proposal sounds a bit unpredictable what exactly you'd expect when copying?

See https://www.mediawiki.org/wiki/Extension:Math/advancedSettings#CSS_for_the_MathML_with_SVG_fallback_mode

I've a bit of javascript which allows copying formula on a double click

jQuery( document ).ready( function( $ ) {
    $(".mwe-math-mathml-a11y").each(function(ind,ele) {
		root = ele.parentElement;
		$(root).dblclick(function(ev) {
			console.log(ev.delegateTarget);
			var root = ev.delegateTarget;
			var anno = $(root).find("annotation");
			var latex = $(root).find("annotation").text();
			var $temp = $("<input>")
			$("body").append($temp);
			$temp.val(latex).select();
			document.execCommand("copy");
			$temp.remove();
		});
});

Vanalla MathJax does provide a right click menu which allows copying an equation. It would no be too tricky to do something similar here.

As to the actual task description, there are three possible options for what should be copied.

1. The latex code
2. The semantic MathML code
3. a translation of the equation into some Unicode type expression.

1 and 2 are easy, the data is already embedded in the HTML source code. 3 Is a much harder task, it might be possible for some simple equations like polynomials, but it gets considerably harder as the size of the expression grows. Defining the desired output for 3 is a task in itself. Wondering if there is some code out there that converts MathML into some useful format.