I think hugostyles had the right idea, but I'm not sure that it would quite achieve what you're trying. I'd suggest doing it like this:
(I'm assuming you're using single track edit mode, you'll have to modify this if you're using A-B edit mode).
1. Make a copy of the clip you want to modify.
2. Paste the copy on the same timeline as the original (you may need to paste a copy of the audio track as well).
3. Change the speed of the original clip to -100%
4. Now sub-divide the two clips into smaller pieces of unequal durations (I'd put cuts 1/2, 3/4 and 7/8s through the orignal, 1/8, 1/4 and 1/2 into the copied clip). You should have 8 parts now.
5. The speed of parts 1 & 8 should remain unchanged, change the rest (in order) to -60, -32, -10, 10, 32 and 60%.
If I read your description correctly, this should achieve the desired result. Each clip (after adjusting the speed) should be roughly the same duration) and the speed changes are closer to logarithmic than geometric so the transition should be more fluid.