Rewriting URLs to single Drupal install in subdirectory

I have been successful in configuring Drupal 5 to live at the root of an account and route multiple sites through the single instance. (I use the same hosting account DNS information for all the domains at my registrar.)

However, I want that instance of Drupal to live in a subdirectory named, say, /drupal/, so the account can be used for other functions, as well.

How can I use an htaccess file to capture the HTTP_HOST information, do an internal rewrite to send those requests to the new directory, and then reinsert the HTTP_HOST back into the request? Then, the user would not see any difference in the URL.

I've done a great deal of searching in Apache and other documents — and found similar solutions for .htaccess rewrite rules in Apache 1.3 — but it appears that I haven't found the correct terminology for a correct answer.

Has anyone done anything like this?