You could say thank you while you have them in contact.
Not every interaction needs commendations.

When I worked at a call centre whenever one of my employees got praise from a customer, all it did was take up my time to sit and listen to someone ramble for 5 minutes about how nice, polite, and understanding they were.
And while yes it's certainly a breath of fresh air from the usual attitude people usually have, it doesn't really do much for the employee. If you really want to help them, if you're sent a survey give them 10 out of 10 or whatever the scale happens to be. That actually directly impacts employees.

I have absolutely no idea how SE's support staff is structured, but the multiple centres I worked for, including online support like a games GM, all worked the same.
I don't mean to diss any GM that likes hearing the positive feedback, as they do absolutely see and respond to these types of posts. And kudos to them when due, it's absolutely nice to see the positive results of your hard work you put in every day. But if you're worried about clogging up a web forum with threads that most people will dismiss and not even pay attention to , my advice to you is to simply not worry about it and carry on with your day. They are still paid for their services, be it a positive experience or negative one. Not every interaction needs to be brought to light.

This probably wasn't the "Yeah lets get a specific section to give feedback" reply you were hoping for, but some insight to how these things work (usually) may be more helpful.