After much struggling with mod_rewrite, I've finally got attemptry working like I want. Drupal is in a subdirectory, but it looks like it's in the root directory.
Links to trac and subversion still work.
Here's the .htaccess file, in case you're interested.
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteRule ^$ /drupal/index.php [L]
RewriteCond %{DOCUMENT_ROOT}/drupal%{REQUEST_URI} -f
RewriteRule .* drupal/$0 [L]
RewriteCond %{REQUEST_URI} !^/trac
RewriteCond %{REQUEST_URI} !^/svn
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule .* /drupal/index.php?q=$0 [L,QSA]
</IfModule>
My only complaint is that it automatically directs you to https. Something to figure out another day.