Hi Ed,
I've tried using a rewrite rule, but (1) I couldn't find an example that exactly matches what I want, and (2) none of the ones I did try worked.
I'm afraid I don't know enough about rewrite rules to properly troubleshoot why it isn't working.
Here's what I'm using:
RewriteEngine on RewriteRule ^wiki/?(.*)$ /w/index.php?title=$1 [L,QSA]
The actual MediaWiki installation resides in /w/, while pretty URLs use /wiki/ (e.g. /wiki/Main_Page etc.). If having different directories there is OK for you, the above or some variation thereof should work.