The 68-pin connectors essentially feedthrough the BNC-2090 and the BNC channels are tied to these pins in parallel. This document explains this in a little more detail:
http://digital.ni.com/public.nsf/websearch/FDE04C8A1E4481B486257199005D71A7?OpenDocument
So you do have access to those pins on the front and the back of the 2090. You just have to be careful that you don't double-drive the same line.
Garrett H
National Instruments
