I think the OP was asking about thread lockers, not sealants. Perhaps he is concerned about having a brake flare nut working loose and allowing the flare seal to leak. He mentioned the through frame fittings (on the inside of the frame) which perhaps could be a bit of work to get to.
I see no problem with using thread locker on the threads making sure the material doesn't contaminate the fluid. I have not used any thread lockers on brake fittings and have never had any come loose. A fastener that has had thread locker applied such as Loctite, when unfastened at some later time, will have to be cleaned up to be re-used, as the thread locker material has a tendency to remain on the threads. Could be a problem on the female side of the fastener.