Malcolm, I'm sure you could easily buy a keeper ring from a decent tackle shop and whip it onto the blank. Since you tie your own flies, this should be well within your skills. You could probably even just glue it with waterproof superglue or epoxy.
Someone once advised me against hooking flies onto the stripping ring, as it may damage the liner. Don't know if there's any truth in this, but on the precautionary principle, I try to remember to hook the fly onto the leg of the ring rather than through the ring itself.
Given the price of Sage rods it does seem pretty naff that they don't give us a keeper ring, though.