I don't know about wine bladders, but when I was teaching photography at a university in the 70's, I used 5 gallon cubes for dektol and fixer. I bought one with the mix in it, then reused over time with a spigot affair. Worked very well, kept dektol for a few months, and was still fairly light in color at the end. But I don't know about the difference in the poly.
Try one (assuming you mean only a gallon), if it goes dark, you'll know that O2 has crept in.