You can add f:param into a4j:support and check value of this parameter at the server. If it is submitted, skip validation.
BTW, if immediate="true" on input is not intentional, you can make a4j:support immediate="true" and input immediate="false" - validation won't be processed too in that case.
immediate = "true" was intentional. I decided to use f:param, like you suggested. Seems like you always have the answer... Thanks again!