I took Roger's advice and bought a Minolta 601-XL with built-in intervalometer. Not sure how it works, though. I assume increasing the intervals makes for slower time lapses. I think this was a good way around the issue.
That's not what he meant. He meant software to alter the running speed in editing. A camera intervalometer does nothing like that- timelapses compress minutes, or hours, into seconds by exposing a frame every few seconds. I don't think that's what you wanted to do at all.
In fact increasing the interval makes time appear to run faster, not slower. At one frame per second, a minute passes in just over 2 seconds at 24fps. At one frame every 10 seconds, five minutes is compressed into just over a second and so on.