JSF: How can I change input field values in a PhaseListener?

Question

I'm trying to use a PhaseListener for the APPLY_REQUEST_VALUES phase for having one central place where I can filter all input fields and sanitize them without adding code to them individually.

In my first step, I've tried just to set one value to a field I left empty in the browser when submitting the form, but I got an Exception telling me that I can't set request parameters.

Below is my experimental PhaseListener. This version tests if I can change any input field content at all by setting one known field to a value.
Once I get that working, the final code shall iterate over all fields, get their content, sanitize it, and write it back to the request.

public class SanitizeRequestValuesPhaseListener implements PhaseListener {
  private static final long serialVersionUID = 1L;

  public static final String SEARCH_FORM_INPUT_PRICE_TO = "searchForm:inputPriceTo";

  public PhaseId getPhaseId() {
    return PhaseId.APPLY_REQUEST_VALUES;
  }

  public void afterPhase(final PhaseEvent event) {
  }

  public void beforePhase(final PhaseEvent event) {
    final FacesContext facesContext = event.getFacesContext();
    final ExternalContext externalContext = facesContext.getExternalContext();

    System.out.println();
    final Map<String, String> requestParameters = externalContext.getRequestParameterMap();
    printMap(requestParameters, "Request Parameters");

    if (requestParameters.containsKey(SEARCH_FORM_INPUT_PRICE_TO)) {
      final String priceTo = requestParameters.get(SEARCH_FORM_INPUT_PRICE_TO);
      if (priceTo != null) {
        // just out of curiosity
        System.out.println(priceTo.getClass().getName());
      }
      try {
        requestParameters.put(SEARCH_FORM_INPUT_PRICE_TO, "120");
      } catch (final Throwable ex) {
        ex.printStackTrace();
      }
    }
  }

  private static void printMap(final Map<?, ?> reqAttrs, final String title) {
    final StringBuilder sb;
    sb = new StringBuilder();
    for (final Object key : reqAttrs.keySet()) {
      sb.append("* (");
      sb.append(key);
      sb.append("=");
      sb.append(reqAttrs.get(key));
      sb.append(")\n");
    }
    System.out.println(title);
    System.out.println(sb);
  }
}

Is there another way to change/override user input values?
Or shoud I use a completely different approach?
Remember: The important thing is to have once central place that handles all input fields across the whole web application.