Boiler water treatment is a contentious subject. I read today that one of the causes of the boiler explosion on the SS France/Norway was inattention to the boiler water, and have experienced similar problems off the coast of Nigeria where the chemicals required were not readily available. The boiler problems of "foaming, priming and carryover" are all conditions of poor boiler water purity.
I guess you know the reasons for adding chemicals and today's methods, and I could give a more concise answer if I had my "1927 Sotherns Marine Engineering" book with me, but unfortunately that's in another country at the moment...
As far as I remember from that book, boiler alkalinity (to reduce the acidic effects of corrosion and scale formation) was accomplished by dosing with sodium hydroxide (caustic soda as it was referred to then) and phosphates (potash in those days, or potassium compounds today) were added to control sludge formations on the tubes that could inhibit heat transfer. It wasn't the more exact science that it is today on modern ships, but the pressure was quite low - 220psi - and I would imagine frequent blowdown of the boilers was necessary to reduce sludge build up. As far as oxygen scavengers go, such as hydrazine, I don't think they were available at that time, and I remember a picture in the book that shows cracks between boiler rivets which exhibited a classic example of caustic embrittlement (from overdosing of caustic soda on the "chuck in a bucket full" principle) but the author did not know this term or its origin at the time! In early ships in my career at sea, and with open feed systems, we did not dose the feed system with hydrazine as it would just oxygen scavenge the air in the engineroom... We had no injectors to put chemicals directly into the boiler feed stream at that time (late 50s and early 60s built ships with low pressure fire-tube boilers). I would imagine that Titanic and her sisters would have gone in for a mechanical boiler clean at regular intervals to compensate for the lack of knowledge on boiler water chemistry at that time.
In the days before flash evaporators, ships I sailed on had a standard seawater/steam evaporator, and to get shot of the scale on the tubes, you fed live steam through it to heat it up nicely, then opened up cold seawater which cracked the scale off the tubes - dramatic! I don't see them doing that with a boiler though...
Stephen