To follow up, it's really simple. You don't need the Map module, just a text file with rewrite directives, and the include directive.
Doh! Not sure why I went the confusing route...
location / {
include /path/to/rewrite_rules_x.txt;
+++++++++++++++++++++++++++++++
cat /path/to/rewrite_rules_x.txt
rewrite /aaa /someotherpage;
rewrite /bbb http://forum.nginx.org;
...