I've spent a few hours looking for a way to inject HTTP response headers in a rewrited URL directly from the Apache configuration.

Here's the trick, in the RewriteRule just set a environment variable, ie: "addheader".
But unfortunately, this one can't be used as-is as a condition in the "Header" directive. In this case you'll need to rely on the presence / absence of the "REDIRECT_addheader" :

RewriteEngine On
RewriteRule ^([A-Z]{2})_([a-z]{2})$  /rewrite.php?a=$1&b=$2 [L,E=addheader:1]
Header set my-header "myvalue" env=REDIRECT_addheader

Here are the slides of the talk I've given at Journées du Logiciel Libre yesterday in Lyon (Download)

Here's a very cool way to parse ini files inside a shell script.
The following snippet will declare variables in the current scope of your script from all the key/values pairs present in the matching section.

#!/bin/bash
CONFIG_FILE="config.ini"
SECTION="section_1"

eval `sed -e 's/[[:space:]]*\=[[:space:]]*/=/g' \
    -e 's/;.*$//' \
    -e 's/[[:space:]]*$//' \
    -e 's/^[[:space:]]*//' \
    -e "s/^\(.*\)=\([^\"']*\)$/\1=\"\2\"/" \
   < $CONFIG_FILE \
    | sed -n -e "/^\[$SECTION\]/,/^\s*\[/{/^[^;].*\=.*/p;}"`