I don't know if ACR / Lightroom does that, but most tools based off DCRAW do. I'd especially recommend Raw Photo Processor
I'm afraid I'm running good ol' Windows so that tool would be only available in a MacOSX vm and I doubt it'd be worth the hassle, esp. since I'd end up having a demosaiced 100mb tif instead of a 20mb raw dng.
Well, there're Windows apps that use DCRAW -- plus DCRAW itself, if you're not afraid of the command line. But that still leaves you with a TIFF instead of a raw DNG, of course.
But I'm hopeful that in LR's raw processing adjusting the exposure doesn't collide with tone curves either applied manually or via picture styles.
Yes, one would hope so. In the past, I know it was emphatically not the case, but that was many moons ago...no idea if they've fixed it.
It'd be pretty simple to test. Do a three-shot bracket. Apply a corresponding exposure compensation to both the over- and under-exposed shots to normalize them to the middle shot -- that is, if you shot at 0, +1, and -1, then expose for 0, -1, and +1 (precisely by the numbers; don't eyeball it). Then compare all three. If they look identical (except for shadow noise and up to a stop of highlights blowing early), then they've fixed that problem. If there's any visible difference between the three, then they haven't.
(I)f I understand you correctly you're saying that if the postprocessing software is capable of operating in raw space iso50 should be equal to ettr @iso100?
The data recorded by the sensor (and, presumably, written to the raw file) is identical for ISO 50 and ISO 100; all that's changed is the meter is told to overexpose by a stop and the raw processing engine is told to underexpose by a stop. So, yes, if you use the same shutter and aperture, ISO 50 is the same as ISO 100 with one stop of digital underexposure (again, assuming the digital underexposure is done properly, in the camera's linear raw space before any other adjustments).
(There might be some subtle advantage to doing it in-camera with ISO 50...Chuck Westfall could shed some light on that. But, if there is, the effect would be very, very subtle.)