Mythbusters made a lead balloon - big enough and with enough helium - it really did take off. The downside? Lead foil is incredibly delicate, even more so than gold foil. Adam spent an awful lot of time with tape patching it up.
So a lead-lined robe (or cape as I have used) would have to be a multi-ply fabric - with the lead foil glued to a liner to keep it from falling apart. Now, being so thin, it wouldn't really be radiation resistant, but it might still be enough to block kryptonite radiation and Kryptonian x-ray vision.
