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Ok so ProxyHTMLURLMap does not work easily. The documentation searching in google is sub par and the examples do not seem to work. Further to that it uses a SAX parser which implies that it expects good xml. I imagine the overhead to be quite high.
Looking around found what looks to be better,
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Have not measure the performance hit yet.
Refernces
http://www.apachetutor.org/admin/reverseproxies - a bit techie, but the most in depth article I have found on reverse proxy.