So you’re half there. I have a ram record machine that picks up the last bar of the 4 bar pattern each time through. When I cue up one of the “drop time” patterns I only have two or 3 trigs on a ram playback machine. I set the retrig to rather fast rate and infinite hold. PLock the sample start then, copy the trig and paste it on the last step of the pattern locking the Start Time to somewhere in the first 25-50%. and volume at 0 to shut down the retriggering
Apply a slide trig on the first step and your start point will move through the sample at a rate slower than the original pattern and you get very rudimentary 90s time stretch 
Same methodology works on Octatrack and Digitakt. It’s a little different on the DT since there’s no retrig in the audio domain but you have sample length the same as OT so setting the sample to loop and length to 0.03 or something gives you a grain. sweep the start position with a one shot LFO and you’re there.
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