Passing data from setup-purchase to post-purchase

[hoodslide]

Long-time listener, first time caller.  I've got a service which 3rd party applications use to initiate a paypal transaction.  I know what the application is at the time I initiate the "setup-purchase()", and would like to store the name or URL of that application somewhere.  Obvious place might be "notify_url", but that's not available in the gateway's details_for(token) because paypal_express.rb's build_setup_request() doesn't think to xml.tag! it.  Adding this one-liner in fixes that:
                xml.tag! 'n2:NotifyURL', options[:notify_url]
 
Hard to imagine that I'm the first person to have this issue.  Is there a "right" way to handle this without patching?

Thanks
  Steve