Just read David Webster's item on water treatment on the information clipboard.
Would it be possible to make up sachets containing a dry mix of soda ash and tannin in the correct amounts for treatment of say 20 gallons of water? We would then just empty one in at each fill. Some enterprising person could make these up with the right amounts for hard, medium and soft water.
Interesting comment on blowing down when firing up - I have done this and it certainly shifts a lot of crud when the water is boiling freely - don't really need much pressure at all.
Sadly, the mixture used must be based on the KIND of hardness in the water. All the major suppliers of such compounds in the 1920s made a point of requiring a sample of the water to be treated before they would suggest a treatment.
A number of Stanley owners in the US have been using tri-sodium phosphate, with good results. I admit that I am not one of them, but I drive a condensing car.
I don't think tannin is a good idea in automobile boilers, acceptable at 150 psi., not at 600.
I always give the boiler a bit of a blow while steaming up. It makes sure the bottom fittings are clear and gets rid of any air in the feedwater automatic loop.