regsub in error response

[Chris Hecker]

I'd like to do a 301 from a.com to b.com, but preserving the whole url, 
but I can't figure out how to do it.  This doesn't work to send it to 
vcl_error:

  error 751 regsub(req.url,"a.com","b.com");

The regsub isn't allowed there.  Can I stuff it in a header that I can 
get in vcl_error?

This is CentOS:
Name       : varnish
Arch       : i386
Version    : 2.0.6
Release    : 2.el5

Thanks,
Chris