OK, it works now thanks to the URL you sent me to which pointed to another page which dealt with variables...
The problem was not one of bracketing but of underscores...
You set a variable with underscores (1 or 2) in front of it to control inheritance but you must not put those in front of the variable name when you retrieve them ("Note that for retrieval purposes these variable names do not include the underscores." from http://www.voip-info.org/wiki/view/Asterisk+variables)...
Thing is, those underscores were there (ie the dial plan is generated that way by FreePBX) before I started playing with this so it was never able to retrieve that variable...
I will check to see if correctly using the variable makes any difference with an almost unmodified dial plan now...
Thank you,
Nick