I want the information from a syntactic predicate but don't want to select an alternative.

You can't do it, but you can probably work around it. I wanted to know if a case block had any labels, if so the translation was slightly different, so I just wanted to set a flag.

caseBlock
  {
    hasLabel = false;
  }
      :
        (#("begin" (~(LABEL))* LABEL))=> {hasLabel = true;}
         #("begin"
           rest of rule here...
      ;

ANTLR complains because you aren't using the syntactic predicate to choose an alternative. What I did was made a seperate rule that does the work of transforming the case block and then passed in a parameter based on the syntactic predicate.

  caseBlock
      :
        (#("begin" (~(LABEL))* LABEL))=> caseBlockTransform[true]
        | caseBlockTransform[false]
      ;