Interesting question - not only relevant to 285s! On my 21 I have a little rudder which reaches down just as far as the bottom of the prop and which I use if the water is too shallow for the proper blade. It pivots on the same bolt as the main blade but is on the outside of the stock cheek.
For an emergency rudder you need to guess what gave way to give rise to the need! The most likely rudder problem would be the failure of the downhaul letting the rudder bob up to horizontal with big forces needed to steer. These might next lead to the tiller breaking off at the stock! Maybe a spare tiller is a more likely need than a spare rudder.
Then there is the bottom pintle attachment to the transom. Mine has started to creak a bit (I think) and a check today showed I could get about one turn up on all 4 of the bolts - not sure what they're screwed into :o(
If that lets go then it's the dinghy oar over the quarter I suppose. If you haven't left them in the dinghy on the mooring as I usually do!
Of all the parts of the Parker rudder arrangement I would have thought the blade would be the least likely to fail.
Geoff Harwood 21/30 Cygnus