While no longer looking for these, here are links to the most promising leads to BNC-sized COTS Twinax connectors:
These connectors from Blackbox, undoubably mfr'd by someone else, are compatible with an old IBM-style serial connector:
http://www.blackbox.com/Store/Detail.aspx/Twinax-Connector-Single/FD010 IBM part# 7362229
http://www.blackbox.com/Store/Detail.aspx/Twinax-Termination-Cap/FD115-R2 Termination cap. Note absolutely no info on what the termination resistance is.
http://www.blackbox.com/Store/Detail.aspx/Twinax-Bulkhead-Connector-Single/FD020 IBM part# 7362229
Amphenol makes a series as well which doesn't look too compatible with the above (but which is available through digikey):
http://drawings.amphenolrf.com//pdf/169.pdf
Digikey pages http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ARFX1035-ND
