Using trial and error I arrived at a javascript function that opens the record as a new page, it works both outside and as an page embedded inside crm 2013.
I'm calling the function with something like this:
openEntityRecord("activity", "1234-45687-6785-2342", "https://contoso.mydomain.com");
Function:
function openEntityRecord(enityLogicalName, guid, baseUrl) {
var randomnumber = 100000000 + Math.floor(Math.random() * 900000000);
var url = baseUrl + "main.aspx?etn=" + enityLogicalName + "&extraqs=&histKey=" + randomnumber + "&id={" + guid + "}&newWindow=true&pagetype=entityrecord";
window.open(url, "", "status=0,resizable=1,width=1000px,height=600px");
}