Jeff,
I haven't done this but it doesn't sound too hard.
1) Write a js snippet that fires off a call to a a custom controller action - let's call it check_user_attribute(). 2) Wire this snippet up with the onblur event handler of form's fields. 3) In check_user_attribute, build user and check valid?, then check errors on 'fieldname'. Update field as needed depending on outcome via rjs.
Note that this is resource-intensive with all the remote calls and running the full validation every time a user changes fields.
