I'm beginning to think this is a bug in the actual Login extra itself. Just to check what was being passed through the script properties, I added a line to /core/components/login/controllers/web/Login.php on Line 337 to log the value of loginResourceParams:
if (empty($loginResourceParams)) { $this->modx->log(modX::LOG_LEVEL_ERROR,'[Login] $loginResourceParams is empty...'); }
When I log in, the error log just says:
[2016-07-05 17:35:37] (ERROR @ /index.php) [Login] $loginResourceParams is empty...
So the value from the snippet is not being passed at all, or so it seems. I am using the most recent version of Login 1.9.2.