There isn't a host header in responses. The app would potentially include absolute references to the hostname in HTTP headers and the payload.
If the ProxyPass iRule is overkill for your scenario, you might be able to take a stripped down version and adapt it for your requirements. This Codeshare example is for rewriting the URI, but you could adapt it for external/internal hostnames in your scenario.
http://devcentral.f5.com/wiki/default.aspx/iRules/Uri_Version_Mapping.html
Here is a version that assumes the application does not include absolute references to the "internal" hostname in response content:
iRule to change host headers
http://devcentral.f5.com/Default.aspx?tabid=53&forumid=5&tpage=1&view=topic&postid=19917
Aaron