Hydraulic clutch fluid

It has merit.
Both are lubes with anti-wear and anti-rust additives.
Hydraulic oil is less expensive than brake fluid. Finding it in a container smaller than a gallon may not be easy, though.
Hydraulic oil isn't as nasty if leaked.
Brake fluid would probably be easier to find in the field, but almost any engine or transmission oil could be used to add to hydraulic oil. On the other hand, I wouldn't want to add any other substance to brake fluid. If I did, I'd want to flush it at the earliest opportunity.
If I were you, I'd use the brake fluid because I already have it.