It may be possible. See
here. Near the end is a callback function that incorporates html:
function authCallback(callbackRequest) {
var authorized = getOAuthService().handleCallback(callbackRequest);
if (authorized) {
return HtmlService.createHtmlOutput(
'Success! <script>setTimeout(function() { top.window.close() }, 1);</script>');
} else {
return HtmlService.createHtmlOutput('Denied');
}
}
Which means, you may be able to replace "
HtmlService.createHtmlOutput("<html>...") with
HtmlService.createHtmlOutputFromFile("your HTML file").
If you have success, please post your results! I'd be interested to see just what you can do.