Is is possible to actually fake your IP details via mechanize? But if it's not, for what is br.set_proxies() used?
You cant' fake you IP address because IP is third layer and HTTP works on 7th layer.
So, it's imposible to send ip with non-your ip.
(you can set up second interface and IP address using iproute2 and set routing throught that interface, but it's not python/mechanize level. it's system level)
Change IP with Python mechanize