Log and suppress proxy header

I'd like to log a header from a proxied (via X-Accel) server with $sent_http_<header> before suppressing it with proxy_hide_header <header>.

Both of these work alone but, as expected proxy_hide_header erases $sent_http_<header>.

Any pointers on how to make this work? I've tried various combinations of variables and sequences of directives without success.

Thanks,

Alex