custom validator webservice call

What you want to do is possible, but I'm not sure using a ASP.NET custom validator is the easiest way to go. If this custom validation is only on one or two pages, I'd just do the validation manually using a callback and some JQuery code.

Add a click event handler to your submit button (and any other button that should cause validation) that looks something like this:

$('.buttonsThatCauseValidation').live("click", function(event) {
    if (event.button != 2) { // Proceed if this is not a right-click
        // Display your progress bar/spinner/modal UI here.
        // Example using the JQuery BlockUI plugin: $.blockUI({ message: '<h1>Please wait while we begin processing your form.</h1>', css: { padding: '10px' } });
        <Insert CallbackReference Here>;
        return false; // Return false here to prevent the button from actually doing the postback or submitting the form.
    }
    return true;  // Return true here to allow the default action to occur.
});

Then in your client-side callback handler you check your validation result and either show an error message or proceed with the postback/submit operation.

function GetValidationCallbackResult(result) {
    // Hide your progress bar/spinner/modal UI here.
    // Example using the JQuery BlockUI plugin: $.unblockUI();
    if(result == "true") {
        <Insert PostbackReference Here>;
    }
    else {
        // Display the appropriate error message.
    }
}

Since you need callback and postback references, this code should be added using Page.ClientScript.RegisterClientScriptBlock().