The method to solve this turns out to be (once again courtesy of MJR) the
following modifications to your game:

modify inspectVerb
    doDefault (actor, prep, iobj) =
        {
        if (objwords(1) = ['A'])
            {
            global.allMessage := 'You can\'t use "all" with this verb.';
            return [];
            }
        pass doDefault;
        }
;

parseError: function (str, num)
    {
    // if there's an allMessage waiting, use it instead of the default
    if (global.allMessage <> nil)
        {
        local r;

        r := global.allMessage;
        global.allMessage := nil;
        return r;
        }
    else
        return nil;
    }