I'm thinking powdercoating..
Originally from the factory the rear subframe was hot-dip galvanised.
You have two different goals here...
If you want to refresh and protect the subframe then the easiest is to bring it to a company that does hot-dip zinc plating. They usually have acid baths to take off any old zinc from material and will then re-dip it for you. Usually guys like these work in hundreds and thousands of kilograms of steel to strip and plate (eg. old fences, industrial carts, etc. etc.) and a tiny subframe won't bother them
That should take care of protecting the subframe for the next 20 years or so. You will need to clean out some threaded holes, but the zinc layer thickness on hot-dip is much bigger than electro-plating, so it lasts a loooong time.
Not sure it will show up, but here's a stripped and re-dipped subframe from a toyota-Elise:
The structure was already designed for hot-dip (no enclosed voids) and an 'after' check on the dimensions on the subframe above showed no distortion compared to pre-dip.
(BTW.. Might also be a good idea to check the lower rear 'legs' of the subframe for crack development and if found have them ground out and welded to fix...)
Second goal is then preventing the galvanic corrosion and white powder developing between the steel subframe and the alu chassis once it's back together.
Solution here will be the proper application of Duralac on the mating surfaces and also using the excellent new coated and shims Gaz has been producing.
Once it's all bolted back together I guess you could douse the joint with a conserving wax type coating (eg. Tectyl, Waxoyl, etc.) to stop moisture creeping in, although the duralac should already basically seal the joint itself once it fully hardens once it's all assembled.