RedirectMap is great, but I needed it to perform a redirect only if the incoming URL was actually in the specified text file. All other incoming URLs should be passed along and handled as normal.

After some digging and an assist from StackOverflow, the key bit ended up being RewriteCond:

RewriteCond "${MyRedirectMap:$1|NOT_FOUND}" !NOT_FOUND
RewriteRule ^(.*)$ "${MyRedirectMap:$1}" [R,L]

Without RewriteCond, every incoming URL is sent to MyRedirectMap which successfully redirects if the URL is found but breaks the site if the URL is not found.

With RewriteCond in place, every incoming URL is first checked for existence in MyRedirectMap and the redirect only occurs if the URL is found.