Unfortunately, I'm fairly sure what you want to do isn't possible: I'm trying to do the exact same thing! From my research, I'm fairly confident it's not possible.
Put simply, you need to use ProxyPassReverse, which is only available at a VirtualHost level (or similar); not a htaccess level.
Edit: the only way I have achieved this is by also configuring the responding server/application to know it's behind a proxy, and serving pages appropriately. That is, I use .htaccess to redirect to another server as follows:
RewriteEngine on
RewriteRule (.*) http://localhost:8080/$1 [P,L]
Then on the application server -- in this case, a JIRA installation -- I configured the Java Tomcat/Catalina appropriately to serve pages with the proxied information:
proxyName="my.public.address.com"
proxyPort="80"
However, that's not completely transparent; the app server needs to serve pages in a proxied manner. It might be of some use, though.
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