Brake fluid is more accurately a hydraulic fluid than a hydraulic oil. Oils are typically composed almost entirely of hydrocarbons whereas fluids used in automotive braking applications are a mixture of glycols, poly-glycols and related ethers owing to their higher boiling points. Hydraulic fluids used in low-temp apps may well be oils; I wouldn't know. Real oils would be great in braking applications since they wouldn't suck up water like the glycol based fluids do were it not for their boiling point limitations.
Bulk fluids for recycling are tested on receipt by the buyer. It doesn't take a lot of brake fluid or antifreeze in a tanker of waste oil to turn a potentially saleable load into a costly disposal burden. Even incinerator operators are extremely careful about what they accept. There are better ways to get rid of brake fluid than mixing it with waste oil. It shouldn't be too hard to find one. I'll look into it and post what I find.