Hi Brian, I'm not sure how you're getting this behavior - reenabling with TS_EVENT_HTTP_ERROR on the TS_EVENT_HTTP_SEND_REQUEST_HDR should not release the headers to the upstream server. The connection should be established and immediately torn down without any data being transmitted by the ATS. (AFAIK this had always been the case, and I've just double checked on latest 6.0.0 with an adjusted bnull-transform plugin).
Are you sure you haven't got an extra reenable(with a CONTINUE) lurking around somewhere? If you can share your plugin code I'll try to take a look..... Cheers, Uri