Mod_rewrite with Proxy [P] flag

EDIT: I did some googling and I found out the answer is straight forward enough: DH doesn’t enable mod_proxy.

Original post below:

Has anyone been able to successfully use the proxy flag ([P]) when writing a mod_rewrite rule? I’m trying to redirect silently from a wildcard ServerAlias.

In other words:
RewriteCond %{HTTP_HOST} ^(www.)?[^.]$ [NC]
RewriteRule ^(.)$ %{HTTP_HOST}/$1 [C]
RewriteRule ^(www.)?([^.] )
)$$2/$3 [P,L]

Which fails with the P flag but succeeds (not invisibly) without it.