sooo by the time it got posted I've sorted it out (Browsed through odoo code) 

here is the working js implementation:

/** @odoo-module */
import publicWidget from "@web/legacy/js/public/public_widget";

publicWidget.registry.WebsiteSale.include({

    /**
         * @override
         */
    _onChangeCombination(ev, $parent, combination) {
        const res = this._super.apply(this, arguments);

        const variantDesc = combination?.website_description_variant || '';
        this.$('#variant_description').html(variantDesc);


        return res;
    },
});

might share a module on github soon