Hi Andy,
You have following options
1) You may ask the network admin to NAT a public IP address to CRM UI URL. Public IP can have some domain name which can be shared with users.
2) Make use of 3rd Party Solution . Refer link http://www.innovate-gmbh.de/98-0-innovate-Starter-SAP-GUI-CRM-Web-UI-CRM-7-0.html
3 ) Make use of Web Dispatcher. Easiest and secure to use. Refer link for its configuration
http://wiki.sdn.sap.com/wiki/display/CRM/CRM+UI+Load+Balancing+With+Webdispatcher
4) You may additionally intergrate CRM-UI to a External facing SAP portal using URL iViews.
Hope this is useful.
Regards,
Deepak Kori