So did you cut your barrel yet ?
I just saw this thread now , wish I'd have seen it earlier.
Before you cut it , drill some 1 inch teck screws thru the hoops and into each wood stave , that will keep it from falling apart when cut.
And yes , I would keep it full of a saturated brine when not in use , if cared for properly , it should work fine.
Actually , some of the worlds best Feta is aged in oak barrels , and the barrels are re-used many times.
Barrel Aged Feta
Barrel Aged Feta is a classic Greek style feta made from a mixture of sheep's and goat's pasteurized milk, produced by Mt. Vikos, Greece. The cheese is aged in 120-pound oak barrels for about 6 months. the cheese is crumbly yet creamy in texture with rich, sweet and tangy flavour.
As I understand , that's how washed curd cheeses came about , as they couldn't heat their wooden vats , so hot water was added to the milk to heat it up.