HA! I had no idea dcraw now supported ACES directly now, that's awesome!
As for the clipping, 16bit TIFF being an integer format is only an issue if you're actually trying to store values above 1, which isn't an issue at this point. It's only when you scale up the "too dark" data.
More or less all RAW formats are 12 or 14bit integer black and white images. There isn't anything "above 1". Highlight recovery is either scaling down values that the default display transform deemed over exposed (but are still under 1 in the original RAW data), or values being interpolated from other unclipped channels.
If you want to get a really clear look at whats actually in your RAW files, use dcraw's "document mode"
-d Document mode (no color, no interpolation)
-D Document mode without scaling (totally raw)
This basically dumps the raw un-debayered sensor data into a tif. From there you can really poke around and see what's going on, even build you're own debayer system from basic nuke nodes. (I'd recommend this as an interesting technical excercise, not a practical solution to debayering images).
As for ACR producing better highlights on the tape, I'd simply put that down to Adobe having more sophisticated highlight recovery. Whilst it's not always the easiest tool to integrate, Lightroom is very impressive on that front.