I'm currently using this solution. Not optimal, but working for my needs.
iframe.onload = continueOnLoadOfSvgInStdIframe;
iframe.src = url;
function continueOnLoadOfSvgInStdIframe() {
var svg = iframe.contentDocument;
var docEl = svg.documentElement;
if (typeof docEl.width==='undefined') { return; }
var w = docEl.width.baseVal.value;
var h = docEl.height.baseVal.value;
docEl.setAttribute("viewBox", "0 0 "+w+ " "+h);
docEl.setAttribute("width",iframe.clientWidth);
docEl.setAttribute("height",iframe.clientHeight);
}