I have noticed that you are quite concerned over use of presoak and you have an impression of presoak being an ACID. No No No. Presoak is not an acid. As it name refers ( PRE SOAK ) means a combination of chemicals to be used before SOAKING (Rinsing) something. Presoak is a fast acting and field tested blend of foaming agents, soil release agents, and safe alkaline cleaning agents cut through and loosen road film. It can work well in the areas which you can not reach with hand.
Presoak is used at car wash stations in foreign countries. It is clear coat safe, so you can well imagine that if the whole car can be washed with this concentrate then it definitely wouldn't harm plastic and rubber parts of the engine.
Just keep in mind following things while using presoaks.
1- Dilute the presoak in water as per manufacturer's specification.
2- Properly cover all electronic parts to avoid water.
3- Thoroughly Rinse the presoak after cleaning.
4- Use plastic/rubber rejuvenators and metal polishes for further enhancement/finishing of engine.
Hope this will help.
O yes these are available in market. They belongs to PROFESSIONAL series of many car care product manufacturers. These are usually packed in big containers and you will have to buy from them as KHULLA PRESOAK just like some of us buy KHULLA Mobil Oil.