Hello, as Brian metnioned, the javascript approach may indeed work in this case. Basically, once the page is loaded and the MVT cookie is set, all conversions are being logged to this MVT variant as well. Therefore if your button (that comes fromt the master page) is tracking conversions, they will be correctly assigned to your MVT variant event thought the button is not part of the actual page template.
The only thing missing is indeed changing the appearence, which can be handled by javascript.
Dave