As to rewritemaps themselves, from memory alone, if it isn't enabled within httpd.confg you can't overwrite that value via .htaccess
That is not my experience - if the Apache instance is not compiled with mod_rewrite, then none of the features work... but I do nothing special to my Apache instances beyond installing mod_rewrite to enable rewrite maps. I use them like a big dog. That being said, I have never used them from htaccess ... since I own all of my boxes I install everything as includes into the httpd.conf.
Are you saying that you'd *like* to cloak via PHP? IF so I can assist as well - since I currently am doing both... I have sites using a rewritemap as well as a front-end php script solution. CAVEAT: I convert the rewritemap to an NDBM type file for speed in Apache, and I use another format saved in RAM and accessed via APC for speed in pure PHP... neither is for the faint of heart at the outset, but I'm sure we can get you going in the right direction.
Agree with Jason that the front end script is easier... but I must admit there is an elegance that still attracts me to the Apache rewrite solution... and Jason, based on your other thread you'll understand even better - I have a legacy site that needed upgrading, but it's a nasty confluence of old Kylix code and web services... today that application is:
* Cloaked via rewrite map: certain addresses get passed onto box(a) which has a php based spider site. If it's not in the rewrite map, processing continues
* Split into two services: if the URL looks like (x) then it is proxied onto box(b) with a PHP framework if not, processing continues
* Load balanced into boxes(c..d) running a Kylix application to handle the remaining request types
rewrite maps do all of that for me and they're fantastic. But my new sites and framework simply rewrite all URLs into a single PHP script and all processing is done there... so the net-net is that I've found single-script processing of everything to be much easier and scalable as well. (Although I still use maps for load balancing).
<edit>When I first started with rewrite maps I had lots of errors that LOOKED like it just wasn't looking... by adding rewrite logging I was able to quickly debug what was <not> happening and repair it</edit>