rewrite base url without using $base_url in settings.php while using subdomain module

My drupal installation is in the subfolder '/drupal'.

I hide the subfolder from the browser URL in .htaccess one folder below /drupal with these lines...
RewriteCond %{HTTP_HOST} ^.+\.myweb\.org$ [NC]
RewriteCond %{REQUEST_URI} !drupal/ [NC]
RewriteRule ^(.*)$ drupal/$1 [NS,L]

This works effectively in all my drupal sites, though I need to also set the base_url in settings.php as follows...
$base_url = 'http://www.myweb.org';

Now my problem is that on my new website I want to use the 'subdomain' module and I want to keep the user on whatever subdomain he used to enter the site. However, I cannot set $base_url in settings.php to do it because the base_url could have any arbitrary subdomain. Currently I can hit my site properly from any 'subdomain', but without the $base_url set all the links on the page default to a base_url of 'http://www.myweb.org/drupal'. That is the base url has www instead of the subdomain and also no longer hides the 'drupal' subfolder. What I want instead is the base_url to be 'http://subdomain.myweb.org' with the same subdomain used to call the page and also hiding the drupal folder.

I am hoping this can be done with the right .htaccess settings or some other fix.

I did uncomment the line 'RewriteBase /' in the .htaccess file at the drupal root directory, but this appeared to effect nothing.

Though this may not effect your answer I am also learning to use the module og - organic groups in combination with subdomain. Perhaps that will have an impace on the solution?

Any ideas?

Jeff