How to set a field to is:valid true or false?

[nrietsch]

I'm facing the same issue as the one reported in elclanrs/jq-idealforms-old #109: Can we SET field to valid / invalid?

I have a username input field where I'm checking if the username entered already exists or not.
There is the ajax extension but I'm using C# and I just can't make it call the code behind.

So, I'm using a 'normal' Ajax call as i usually do but how can I then set to field to valid (i.e. the username entered is available) or invalid (i.e. the username is not available)?