the sync switch off after 1sec in the lenses....
Well if you think about the original purpose of synch it is to fire a flash so why would a designer feel the need to keep the contact 'closed'
Another try is if you have a second body is to plug the lead into that one and fire it that way (old lense attached) !!
Morgan, are you sure of what you said there ?
I don't know about those particular lenses, but I used to do a lot of long time exposure with attached flash synch, and never run into that problem.
All the lenses I have used so far, LF w/ different shutters, Mamiya, Hasselblad, a few 35mm systems, etc. do nothing but produce a very brief initial short of the synch, after that the the contact is opened immidiately ( --> no further synchronizing).
If that wouldn't be the case, a connected flash would just keep firing till the shutter is closed, as is the case when you shorten the synch cabel manually, e.g. with a screw driver, and keep it on the contacts.
Hence I assume those lenses send a second signal through the synch after 1 second, closing down the digital back. Just an assumption; but I can't believe the initial signal lasts longer than a split second with any lens.