I tried to use BulkSendApplication with NSC and found that it stalled as soon as the send buffer initially filled up.

BulkSendApplication::DataSend() is not being called by the NSC implementation.  I do not yet know whether an easy fix is to tie this to hook the callback to NotifyDataSent (which is called when the NSC glue code performs a write on the underlying NSC socket):

      Simulator::ScheduleNow (&NscTcpSocketImpl::NotifyDataSent, this, ret);

or whether the fix requires more work (e.g. checking return value of 
ret = m_nscTcpSocket->send_data ((const char *)buf, size);
and acting accordingly).