Can't get redirects to work in page initialization script to fix JX migration issue

We recently migrated to Khoros from JX (aka Jive-x) and have found that most (if not all) external links within messages are broken because JX routed external links through a redirect mechanism via URL manipulation like this:  https://community.example.com/external-link.jspa?url=https:%2F%2Fgoogle.com 

Unfortunately the URLs are all broken and route to the Page Not Found page.

I was hoping I would be able to use the Page Initialization Script to redirect the the URLs to the correct destinations with the FreeMarker code below, but for some reason the code seems to have no effect at all.  (I'm not even seeing anything in the Toolbox.)

<#-- Redirect URLs leveraging the Jive external link redirect -->
<#if (http.request.url)?contains('external-link.jspa')>
  <#assign jiveExternalUrl = (http.request.url)?split('external-link.jspa?url=')[1] />
  <#assign jiveExternalUrl = jiveExternalUrl?replace('%2F', '/') />
  <#assign jiveExternalUrl = jiveExternalUrl?replace('%3A', ':') />
  <#assign jiveExternalUrl = jiveExternalUrl?replace('http:', 'https:') />
  ${http.response.setRedirectUrl(jiveExternalUrl)}
</#if>

Any ideas what I'm doing wrong here, or is there a different way that I should be going about what I'm trying to do?

Thanks!!