No, there is no "easy" way. Your best bet would be to do a loop where you first check each previous sibling, then move to the parent node and all of its previous siblings.
You'll need to break the selector into two, 1 to check if the current node could be the top level node in your selector, and 1 to check if it's descendants match.
Edit: This might as well be a plugin. You can use this with any selector in any HTML:
(function($) {
$.fn.closestPrior = function(selector) {
selector = selector.replace(/^\s+|\s+$/g, "");
var combinator = selector.search(/[ +~>]|$/);
var parent = selector.substr(0, combinator);
var children = selector.substr(combinator);
var el = this;
var match = $();
while (el.length && !match.length) {
el = el.prev();
if (!el.length) {
var par = el.parent();
// Don't use the parent - you've already checked all of the previous
// elements in this parent, move to its previous sibling, if any.
while (par.length && !par.prev().length) {
par = par.parent();
}
el = par.prev();
if (!el.length) {
break;
}
}
if (el.is(parent) && el.find(children).length) {
match = el.find(children).last();
}
else if (el.find(selector).length) {
match = el.find(selector).last();
}
}
return match;
}
})(jQuery);