Along the lines of the other responses. If you set up a controller defining the path in your routes.rb of site A, you can use the generated url helpers. Just override the :host as an argument.
Example:
Site A Routes.rb:
...
map.resource whatever
...
Site A Controller:
...
redirect_to edit_whatever_url(:host => "www.siteB.com", :my_parameter => 123)
...
So long as SiteB's web server (rails or otherwise) recognizes the http://www.siteB.com/whaterver/edit?my_parameter=123
you're good.
Caveat: Keep in mind that redirecting a Post with 302 has specific consequences as defined in RFC 2616. In a nutshell it means that a user will be asked to reconfirm their post to the new URL, before the redirected post can succeed.