I got my answer from the GAS community on Google Plus, so credits to the posters there.
https://plus.google.com/106333172328928589411/posts/7g3Vu7iFZfb
Sergii: Check out this gist that shows how to do 2-legged OAuth authentication in GAS https://gist.github.com/rcknr/c5be4eb80d821158c8ef.
Using 2 Legged Oauth you can get access to the ATOM feed of other users:
A piece of working code for it:
function gmail2lo(user) {
var OAUTH_CONSUMER_SECRET = 'secret';
var domain = 'domain'; //use the domain as key in apps panel
var username = 'user';
var xuser = username+'@'+domain;
var method = "GET";
var baseUrl = "https://mail.google.com/mail/feed/atom";
var timestamp = Math.round(new Date().getTime() / 1000);
var paramsJson;
var paramsOauth = {
oauth_consumer_key : domain,
oauth_nonce : timestamp,
oauth_signature_method : "HMAC-SHA1",
oauth_timestamp : timestamp,
oauth_version : "1.0",
'xoauth_requestor_id' : xuser
};
var paramsStringArray = [];
for (var k in paramsJson)
paramsStringArray.push(k + '=' + encodeURIComponent(paramsJson[k]));
var oauthStringArray = [];
for (var k in paramsOauth)
oauthStringArray.push(k + '=' + encodeURIComponent(paramsOauth[k]));
var paramsString = paramsStringArray.concat(oauthStringArray).sort().join('&');
var signatureBaseString = method +"&"+ encodeURIComponent(baseUrl) +"&"+ encodeURIComponent(paramsString);
var signatureBytes = Utilities.computeHmacSignature(Utilities.MacAlgorithm.HMAC_SHA_1, signatureBaseString, OAUTH_CONSUMER_SECRET+'&');
var signature = encodeURIComponent(Utilities.base64Encode(signatureBytes));
var xoauthString = 'OAuth ' + oauthStringArray.sort().slice(0,oauthStringArray.length-1).join(", ") + ', oauth_signature=' + signature;
var ooptions = {
headers : {authorization: xoauthString}
}
url = baseUrl;
url += "?" + paramsStringArray.join("&") + '&xoauth_requestor_id=' + encodeURIComponent(xuser);
var response = UrlFetchApp.fetch(url, ooptions).getContentText();
}