Arcane, mechanical self-timers can be found on the second hand market that will trip the 6x7/67, probably also the 645. They are plentiful. Pentax at one stage made its own for the 67. The thing to watch out for is to ensure the plunger is only long enough to trip the shutter and no more (both the 6x7 and 67 bodies have short-throw triggers, thus cable releases/timers with long-throw plungers should be used with caution), as an excessively long plunger could potentially jam the shutter; conversely, a short one (not very common) may bottom out without triggering. Unfortunately the only way to find out what works is buy one!
Have a look here: http://www.ebay.com.au/sch/i.html?_s...mer%26_arr%3D1