I found a way. Never used "set" variables before... need to study nginx more :P

Just before the rewrites i'm saving the original $uri to a variable
[code]
set $cleanuri $uri;
[/code]
which in turn i pass to the proxy via headers
[code]
proxy_set_header X-Rewrite-FullURI $request_uri;
proxy_set_header X-Rewrite-CleanURI $cleanuri;
[/code]

p.s: all my content rewrites are using "last";