Might be a bit soft, though a work hardened grade might be available. I would definately check the tensile stresses in the bolt vs the strength of the banjo. If the force required to seat the copper sealing washers is greater than that required to stretch the bolt, the bajo fitting wont seal.
SS fasteners can foul just as easily on assembly as disassembly...I never assemble SS cap screws without some form of lube, never! In some applications where lubricant may interact with the product, O2 etc., several forms of wax can be used.
SS banjo bolt? I don't see a problem as torque is not an issue in most cases---copper or aluminum sealing washers are rather soft. I do see one issue, that of loosening in vibration service. Not a common problem but, none the less...a chemical locking agent would cure the problem as well as lube the assembly, but dissambly could be a problem.
If you check out manufacturers like Goodridge and Hel who make braided brake lines for motorcycles etc you will see that stainless is used. One company even quoted using 303.
