Per Jim at webmasterworld (thanks!)
"The [P] flag invokes a reverse-proxy request to the server at the designated URL; That is, it opens a new out-going HTTP connection and sends a request to that server. So at the very least, your configuration is twice as slow as it should be, just using the original working rule, because your server is sending itself a new request via HTTP instead of just serving the content from a non-default-mapped filepath.
It seems to me that all that's needed is an internal rewrite, so that requests for the resource at URL
http://dev.example.edu/distribution/ are served with the content generated by the script at the server filepath /distribution/index.php/distribution/"
RewriteEngine on
#
# Return 403-Forbidden response to TRACE requests
RewriteCond %{REQUEST_METHOD} ^TRACE
RewriteRule .* - [F]
#
# Internally rewrite requests for URL-path /recreation/<anything>
# to filepath /eel/index.php/recreation/<anything>
RewriteCond %{HTTP_HOST} ^dev\.example\.edu [NC]
RewriteRule ^/recreation/(.*)$ /ee1/index.php/recreation/$1 [L]
#
# Internally rewrite requests for URL-path /distribution/<anything>
# to filepath /distribution/index.php/distribution/<anything>
RewriteCond %{HTTP_HOST} ^dev\.example\.edu [NC]
RewriteRule ^/distribution/(.*)$ /distribution/index.php/distribution/$1 [L]
So I think I was just making it more complicated than it had to be. I've removed the P flag and removed the full server address from the rewriterule.