I can't find a study that fits your description, but I think I may have found a study similar to what you were looking for. Some unpublished preliminary work by the University of Windsor on the relationship between rate of concussions in football players and playing in a hot environment (and thus a dehydrated state), researchers found that "the relationship between hydration status and injury occurrence, such as skeletal muscle cramps, is well known, but more information is needed regarding physiological status and the risk of more severe injury like concussion. We were initially interested in the link between dehydration and concussion frequency/severity, however this type of observation was obviously problematic (e.g., measuring game time hydration status in thousands of athletes prior to head injury). Hydration status was unknown, and factors like helmet function (which is compromised in cold conditions) may hide true differences."

As they explain themselves, this is a study (not an experiment) that will require massive amounts of data collection before valid conclusions can be drawn. One of the biggest issues that I have is they keep repeating that at dehydration causing 2% body weight loss (which is definitely significant), cerebrospinal fluid (CSF) volume is decreased. I can't find this claim repeated in any other peer-reviewed studies - just random blogs and "news" postings online.

Another study I found on PubMed shows that CSF volume in sheep is maintained during mild dehydration. Key things to consider are that these are sheep and that dehydration is "mild", meaning over at 48 hour period.

A few more studies that are blocked behind paywalls hypothesize hypotension as a mechanism to explain headaches due to changes in intracranial pressure.

Overall, I can't find anything conclusive that says dehydration at X% causes Y% decrease in CSF volume. I only looked for about 30 min, so if you want to keep looking please send me some links. I think the biggest factor to consider is that CSF is produced from arterial blood plasma. So when you are severely dehydrated, CSF production may decrease in extreme cases. This is not to say that you are losing CSF volume, it will just be produced at a slower rate. I'm certainly not a neurologist, but my guess is that if you are dehydrated to the point where this is occurring, you won't be in any condition to be fighting someone - you'll probably be in the hospital on an IV with much more serious issues, like organ failure, arising.