Well, no answers but some more research pointed me to https://developers.google.com/google-apps/spreadsheets/#working_with_list-based_feeds where clicking under the 'protocol' link of 'adding a list row' gave me enough clues to construct the following:
//create xml with each element representing a column header (note: for some reason the headers must only contain alphanumeric characters)
$xml = "
<entry xmlns='http://www.w3.org/2005/Atom'
xmlns:gsx='http://schemas.google.com/spreadsheets/2006/extended'>
<gsx:id>123</gsx:id>
<gsx:status>123</gsx:status>
<gsx:date>4/26/2014</gsx:date>
<gsx:user>bob</gsx:user>
</entry>";
//set headers which includes auth from elsewhere in the code
$headers = array(
"Authorization: GoogleLogin auth=" . $auth,
"GData-Version: 3.0",
"Content-Type: application/atom+xml",
);
//post the xml. The key in the url is taken from the actual url when you view the sheet
$curl = curl_init();
curl_setopt($curl, CURLOPT_POST, 1);
curl_setopt($curl, CURLOPT_URL, "https://spreadsheets.google.com/feeds/list/xxxxxxxxxxxxxxxkeyxxxxxxxxx/0/private/full");
curl_setopt($curl, CURLOPT_HTTPHEADER, $headers);
curl_setopt($curl, CURLOPT_POSTFIELDS, "$xml");
curl_setopt($curl, CURLOPT_FOLLOWLOCATION, true);
curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, 1);
$response = curl_exec($curl);
Hopefully this helps someone else