Attached Link: http://www.vmall.ne.jp/~yoichiro/program/mirror_wgt/index.html

Thanks to everyone for these suggestions,

I indeed found the mirror weight plugin (actually in a post to Renderosity LW Forum) : it can be found via the attached link

This already saves me 50% of the work! ;-)

Now, if anyone is interested by jamball77's suggestion (build the map collectively) please contact me.

In the meantime, I will try to start the job on my own and see if the result is worth it...

Cheers.