Is it your own server or will this be happening in htaccess? RewriteMaps can only be used in the config file :(
If is is htaccess, there are two practical approaches. If there are just a few URLs involved, make a separate rule for each page. If there are lots of them, rewrite to a php script that does everything and finally issues the redirect.
Yes, you can make RewriteRules that take a randomly cased URL and make it all lower-case using 26 rules in combination with a string of [N] flags. But unless your name is jdmorgan, I wouldn't recommend it.