I am sorry ahead of time if this sounds like too lengthy. I figure more detail is preferable to less and hope I am not horribly rambling :-)

I personally use WAMP on my small laptop for local dev, and that i have various c:/wamp/alias/* files each pointing to some project working directory. I have had some excellent mod_rewrite help and become

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*\/)?ih(\/.*)?$ $1index.php$2 [L,QSA]

to operate to alter localhost/.../ih/sub/dir to localhost/.../index.php/sub/dir for Web addresses which are both Search engine optimization-friendly and short. [I've not gone onto do that in push however i suspect it'll work equally well.] However, to have it altogether I needed to change my doc root from c:/wamp/www/ to c:/, which I'd really rather not just do just in case my Apache will get compromised and otherwise since it is a kludge.

My test alias file appears like

Alias /testme "c:/var/tmp/wamp-testme/"
<Directory "c:/var/tmp/wamp-testme/">
  Options Indexes FollowSymLinks MultiViews
  AllowOverride all
    Order Deny,Allow
    Deny from all
    Allow from
  RewriteEngine On
  RewriteCond %{REQUEST_FILENAME} !-f
  RewriteCond %{REQUEST_FILENAME} !-d
  RewriteRule ^(.*\/)?ih(\/.*)?$ $1index.php$2 [L,QSA]

and also the error I recieve when attemping to load http://localhost/testme/rewrites/ih/sub/path is

[Thu Jun 30 06:46:13 2011] [error] [client]
  File does not exist: C:/wamp/www/var

having a matching

Not Found
The requested URL /var/tmp/wamp-testme/rewrites/index.php/sub/path
  was not found on this server.

within the browser. Affirmed, exactly the same config during my c:/wamp/alias/flying.conf file that points towards the c:/data/flying/ directory throws File does not exist: C:/wamp/www/data within the error log file, and so forth.

Sooooo... How do i possess a rewrite rule that goes beyond aliases without getting my doc root inside my machine root dir?

Odd: try adding a PT flag towards the rewrite rule: this forces the re-written Hyperlink to be delivered back towards the URL mapping engine.

EDIT: try setting the RewriteBase around the Directory.