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;
...