These suggestions were helpful. Thank you! Using deltas for x,y,z did help quite a bit.
The time delta derivative approach was interesting to think about, but would basically break the lossless compression I have going. At the moment, at least, I need to be able to reconstruct the original input verbatim, and the only way to do that is with fixed point arithmetic.
The one concession I was able to safely (verifiably) make is quantizing the time deltas that are close to the sensor frequency. The clock has a large (+/- 15%!) but predictable jitter pattern I was able to isolate and then basically RLE the result.
With the latest optimizations, I'm sitting at about 9:1 compression, which is acceptable. I was shooting for 10:1, but that last 33 KiB won't come easy.