Great write up with excellent pictures!
Having taken apart three different S4 power steering racks while trying to depower a rack for myself I've got a couple of comments to add.
wire clip thingy that holds the rack stopper in place - Of my three racks I didn't have a single one come out cleanly even after working them back and fourth. Two broke outright and one was heavily mangled. This part is not available from Mazda by itself. I noticed in Stephen's pictures that it looks like he is using a new straight one when reassembling. It appears that a new one comes with the rebuild kit which is quite handy otherwise you're going to have to find some solid core wire of similar diameter to use.
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This is a good time to mention that the keen observer will have noticed a needle bearing in the pinion housing bore. Leave it alone under every circumstance possible. If it's damaged then you have to replace it but if it's fine don't mess with it. They are hard to remove. You can submerge this thing and clean it all you want and not ruin that bearing if you dry everything when your done with air or heat or whatever you have.
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FOR THE LOVE OF GOD, DO NOT DAMAGE THIS!!!! (And if it's already damaged i.e. rusted start looking for a new rack.) The bearing can not be bought from Mazda (doesn't have a part number or even a D-code on the parts diagram) and I couldn't find a single bearing supply house that could get the part and I looked for months. Part number is Koyo BT167. Said bearing supply houses seem to think it's only available for purchase by Mazda. I didn't have any luck finding an alternate supplier either as the height of the bearing is a non standard roller needle bearing height (7/16 inch). I was able to find the specs for it in a Koyo bearing catalog however. Obviously the bearing must be available somehow/somewhere for use in the refurbishing industry but I had no luck finding it. If anyone is able to locate the bearing please let everyone know. All other bearings in the rack are readily available from both Mazda and bearing supply houses. Smaller bearing that goes on the far side of the pinion assembly is Koyo 6001 and Mazda part number FB05-32-147. I don't have the Koyo number for the larger one that goes on the upper part of the pinion assembly but it's Mazda number is FB05-32-115.
An alternative to Purple Power or Simple Green that is friendly to Aluminum would be tide washing detergent (powdered seems to work best) or Pine-Sol. We have had very good luck using both at our shop to degrease aluminum bits.
Other notes -
When you open up the rack if you notice any rust in any of the bearings, the pinion assembly, or shiny parts of the rack you most likely will need to start looking for a new rack. One that i disassembled clearly had had torn boots for a long time and every polished surface that mated to a bearing showed heavy signs of wear, pitting, and discoloration. Even the gear on the pinion assembly was heavily rusted.
If have to put aside the project for any length of time, coat the rack in motor oil/grease/or some other type of petroleum lubricant and wrap it up tight in a garbage bag with some extra lube thrown in for good measure. They will spot rust if left uncoated and sitting around even if taken inside. I'm sure you could carefully clean the rust away with an emery cloth but why risk it. Do the same thing with the pinion assembly.
Inspect the support yoke and rake carefully when you dissemble. Two of the three I have taken apart showed extensive wear to the copper insert on the support yoke and show corresponding signs of wear on the rack. If you show any amount of wear go ahead and replace the support yoke. Part number is FB05-32-111 and should run you about $20. I would suspect that in a depowered application that you would see increased wear on this part too.